$SIGI
Selective Insurance Group Inc Stock $sigi

$sigi Overview Financials Valuation SEC Filings Company Website

2021 Q1 Form 10-K Financial Statement

#000023055721000009 Filed on February 12, 2021

Income Statement

Concept	2021 Q1	2020 Q4	2019 Q4
Revenue	$803.9M	$798.4M	$728.9M
YoY Change	20.92%	9.54%	13.36%
Cost Of Revenue
YoY Change
Gross Profit
YoY Change
Gross Profit Margin
Selling, General & Admin	$9.600M	$6.100M	$2.900M
YoY Change	-20.0%	110.34%	-14.71%
% of Gross Profit
Research & Development
YoY Change
% of Gross Profit
Depreciation & Amortization	$149.1M	$15.00M	$12.90M
YoY Change	9.19%	16.28%	30.3%
% of Gross Profit
Operating Expenses	$660.9M	$631.9M	$620.2M
YoY Change	3.06%	1.89%	6.05%
Operating Profit
YoY Change
Interest Expense	-$7.400M	-$7.500M	-$7.400M
YoY Change	-2.63%	1.35%	21.31%
% of Operating Profit
Other Income/Expense, Net
YoY Change
Pretax Income	$135.6M	$159.0M	$101.3M
YoY Change	747.86%	57.01%	94.29%
Income Tax	$26.36M	$32.00M	$19.40M
% Of Pretax Income	19.44%	20.12%	19.15%
Net Earnings	$109.3M	$127.1M	$81.86M
YoY Change	617.18%	55.22%	78.89%
Net Earnings / Revenue	13.59%	15.91%	11.23%
Basic Earnings Per Share	$1.78	$2.12	$1.38
Diluted Earnings Per Share	$1.77	$2.10	$1.36
COMMON SHARES
Basic Shares Outstanding	59.88M shares	59.83M shares	59.40M shares
Diluted Shares Outstanding

Balance Sheet

Concept	2021 Q1	2020 Q4	2019 Q4
SHORT-TERM ASSETS
Cash & Short-Term Investments
YoY Change
Cash & Equivalents	$8.916M	$15.23M	$7.975M
Short-Term Investments
Other Short-Term Assets
YoY Change
Inventory
Prepaid Expenses
Receivables
Other Receivables
Total Short-Term Assets
YoY Change
LONG-TERM ASSETS
Property, Plant & Equipment	$76.10M	$77.70M	$77.41M
YoY Change	-4.95%	0.37%	18.64%
Goodwill	$7.849M	$7.800M	$7.800M
YoY Change	0.0%	0.0%	-0.62%
Intangibles
YoY Change
Long-Term Investments	$7.559B	$7.506B	$6.689B
YoY Change	10.48%	12.21%	12.21%
Other Assets	$8.400M	$14.80M	$14.50M
YoY Change	-82.61%	2.07%	-79.29%
Total Long-Term Assets
YoY Change
TOTAL ASSETS
Total Short-Term Assets
Total Long-Term Assets
Total Assets	$9.849B	$9.688B	$8.797B
YoY Change	9.73%	10.13%	10.62%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable
YoY Change
Accrued Expenses	$83.70M	$156.5M	$154.3M
YoY Change	-19.05%	1.43%	32.22%
Deferred Revenue
YoY Change
Short-Term Debt	$0.00	$0.00	$0.00
YoY Change	-100.0%
Long-Term Debt Due
YoY Change
Total Short-Term Liabilities	$125.7M	$170.5M	$157.3M
YoY Change	-69.91%	8.39%	33.31%
LONG-TERM LIABILITIES
Long-Term Debt	$550.9M	$550.7M	$550.6M
YoY Change	0.05%	0.02%	25.28%
Other Long-Term Liabilities	$376.0M	$322.0M	$304.0M
YoY Change	28.72%	5.92%	9.51%
Total Long-Term Liabilities	$926.9M	$872.7M	$854.6M
YoY Change	9.99%	2.12%	19.17%
TOTAL LIABILITIES
Total Short-Term Liabilities	$125.7M	$170.5M	$157.3M
Total Long-Term Liabilities	$926.9M	$872.7M	$854.6M
Total Liabilities	$7.105B	$6.949B	$6.602B
YoY Change	3.29%	5.25%	7.16%
SHAREHOLDERS EQUITY
Retained Earnings	$2.363B	$2.272B	$2.081B
YoY Change	13.43%	9.18%	11.95%
Common Stock	$655.0M	$647.1M	$625.5M
YoY Change	3.15%	3.45%	4.95%
Preferred Stock
YoY Change
Treasury Stock (at cost)	$608.7M	$599.9M	$592.8M
YoY Change	1.5%	1.19%	1.4%
Treasury Stock Shares			44.02M shares
Shareholders Equity	$2.744B	$2.739B	$2.195B
YoY Change
Total Liabilities & Shareholders Equity	$9.849B	$9.688B	$8.797B
YoY Change	9.73%	10.13%	10.62%

Cashflow Statement

Concept	2021 Q1	2020 Q4	2019 Q4
OPERATING ACTIVITIES
Net Income	$109.3M	$127.1M	$81.86M
YoY Change	617.18%	55.22%	78.89%
Depreciation, Depletion And Amortization	$149.1M	$15.00M	$12.90M
YoY Change	9.19%	16.28%	30.3%
Cash From Operating Activities	$130.3M	$173.1M	$161.3M
YoY Change	231.3%	7.32%	-0.68%
INVESTING ACTIVITIES
Capital Expenditures	$4.561M	-$4.200M	-$6.600M
YoY Change	-45.81%	-36.36%	34.69%
Acquisitions
YoY Change
Other Investing Activities	-$106.4M	-$186.0M	-$146.0M
YoY Change	-65.39%	27.4%	1.81%
Cash From Investing Activities	-$110.9M	-$190.2M	-$152.6M
YoY Change	-64.88%	24.64%	2.9%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net	$8.845M
YoY Change	27.67%
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	-$25.64M	16.40M	-10.80M
YoY Change	-109.05%	-251.85%	8.0%
NET CHANGE
Cash From Operating Activities	$130.3M	173.1M	161.3M
Cash From Investing Activities	-$110.9M	-190.2M	-152.6M
Cash From Financing Activities	-$25.64M	16.40M	-10.80M
Net Change In Cash	-$6.315M	-700.0K	-2.100M
YoY Change	-195.23%	-66.67%	-151.22%
FREE CASH FLOW
Cash From Operating Activities	$130.3M	$173.1M	$161.3M
Capital Expenditures	$4.561M	-$4.200M	-$6.600M
Free Cash Flow	$125.7M	$177.3M	$167.9M
YoY Change	306.77%	5.6%	0.36%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2020	dei	Amendment Flag AmendmentFlag	false
CY2020	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2020
CY2020	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
CY2020	dei	Entity Central Index Key EntityCentralIndexKey	0000230557
CY2020	dei	Document Type DocumentType	10-K
CY2020	dei	Document Annual Report DocumentAnnualReport	true
CY2020	dei	Document Period End Date DocumentPeriodEndDate	2020-12-31
CY2020	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2020	dei	Document Transition Report DocumentTransitionReport	false
CY2020	dei	Entity File Number EntityFileNumber	001-33067
CY2020	dei	Entity Registrant Name EntityRegistrantName	SELECTIVE INSURANCE GROUP, INC.
CY2020	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	NJ
CY2020	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	22-2168890
CY2020	dei	Entity Address Address Line1 EntityAddressAddressLine1	40 Wantage Avenue
CY2020	dei	Entity Address City Or Town EntityAddressCityOrTown	Branchville
CY2020	dei	Entity Address State Or Province EntityAddressStateOrProvince	NJ
CY2020	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	07890
CY2020	dei	City Area Code CityAreaCode	973
CY2020	dei	Local Phone Number LocalPhoneNumber	948-3000
CY2020	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	Yes
CY2020	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
CY2020	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2020	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
CY2020	dei	Entity Filer Category EntityFilerCategory	Large Accelerated Filer
CY2020	dei	Entity Small Business EntitySmallBusiness	false
CY2020	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
CY2020	dei	Icfr Auditor Attestation Flag IcfrAuditorAttestationFlag	true
CY2020	dei	Entity Shell Company EntityShellCompany	false
CY2020Q2	dei	Entity Public Float EntityPublicFloat	3092111015	USD
CY2021Q1	dei	Entity Common Stock Shares Outstanding EntityCommonStockSharesOutstanding	59882189	shares
CY2020Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	18001000	USD
CY2019Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	21975000	USD
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CY2020Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	-22000	USD
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CY2019Q4	us-gaap	Cash Cash	300000	USD
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CY2018	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	3.00
CY2020	us-gaap	Net Income Loss NetIncomeLoss	246355000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	271623000	USD
CY2018	us-gaap	Net Income Loss NetIncomeLoss	178939000	USD
CY2020	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	133104000	USD
CY2019	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	168021000	USD
CY2018	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	-97284000	USD
CY2020	sigi	Non Credit Portionof Other Than Temporary Impairments Recognizedin Other Comprehensive Income NonCreditPortionofOtherThanTemporaryImpairmentsRecognizedinOtherComprehensiveIncome	-6459000	USD
CY2019	sigi	Non Credit Portionof Other Than Temporary Impairments Recognizedin Other Comprehensive Income NonCreditPortionofOtherThanTemporaryImpairmentsRecognizedinOtherComprehensiveIncome	0	USD
CY2018	sigi	Non Credit Portionof Other Than Temporary Impairments Recognizedin Other Comprehensive Income NonCreditPortionofOtherThanTemporaryImpairmentsRecognizedinOtherComprehensiveIncome	0	USD
CY2020	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Netof Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax	-19000	USD
CY2019	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Netof Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax	-46000	USD
CY2018	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Netof Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax	87000	USD
CY2020	sigi	Oci Reclassification Adjustment From Aoci For Write Down Of Securities Net Of Tax OCIReclassificationAdjustmentFromAOCIForWriteDownOfSecuritiesNetOfTax	4247000	USD
CY2019	sigi	Oci Reclassification Adjustment From Aoci For Write Down Of Securities Net Of Tax OCIReclassificationAdjustmentFromAOCIForWriteDownOfSecuritiesNetOfTax	530000	USD
CY2018	sigi	Oci Reclassification Adjustment From Aoci For Write Down Of Securities Net Of Tax OCIReclassificationAdjustmentFromAOCIForWriteDownOfSecuritiesNetOfTax	31316000	USD
CY2020	sigi	Debt Securities Available For Sale Credit Loss Expense DebtSecuritiesAvailableForSaleCreditLossExpense	3984000	USD
CY2019	sigi	Debt Securities Available For Sale Credit Loss Expense DebtSecuritiesAvailableForSaleCreditLossExpense	0	USD
CY2018	sigi	Debt Securities Available For Sale Credit Loss Expense DebtSecuritiesAvailableForSaleCreditLossExpense	0	USD
CY2020	sigi	Total Unrealized Gains Losseson Investment Securities TotalUnrealizedGainsLossesonInvestmentSecurities	134857000	USD
CY2019	sigi	Total Unrealized Gains Losseson Investment Securities TotalUnrealizedGainsLossesonInvestmentSecurities	168505000	USD
CY2018	sigi	Total Unrealized Gains Losseson Investment Securities TotalUnrealizedGainsLossesonInvestmentSecurities	-65881000	USD
CY2020	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Net Of Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax	1197000	USD
CY2019	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Net Of Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax	-10898000	USD
CY2018	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Net Of Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax	-8906000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax	-2382000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax	-2099000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax	-1680000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	-3579000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	8799000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	7226000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	138436000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	159706000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-73107000	USD
CY2020	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	384791000	USD
CY2019	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	431329000	USD
CY2018	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	105832000	USD
CY2020Q4	us-gaap	Stockholders Equity StockholdersEquity	2738889000	USD
CY2019Q4	us-gaap	Stockholders Equity StockholdersEquity	2194936000	USD
CY2018Q4	us-gaap	Stockholders Equity StockholdersEquity	1791802000	USD
CY2020	us-gaap	Preferred Stock Dividends Per Share Declared PreferredStockDividendsPerShareDeclared	0
CY2019	us-gaap	Preferred Stock Dividends Per Share Declared PreferredStockDividendsPerShareDeclared	0
CY2018	us-gaap	Preferred Stock Dividends Per Share Declared PreferredStockDividendsPerShareDeclared	0
CY2020	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	0.94
CY2019	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	0.83
CY2018	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	0.74
CY2019Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2018Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2017Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2020Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	8000	shares
CY2019Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2018Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2019Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	59461153	shares
CY2018Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	58948554	shares
CY2017Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	58495122	shares
CY2020	us-gaap	Stock Issued During Period Shares Dividend Reinvestment Plan StockIssuedDuringPeriodSharesDividendReinvestmentPlan	28890	shares
CY2019	us-gaap	Stock Issued During Period Shares Dividend Reinvestment Plan StockIssuedDuringPeriodSharesDividendReinvestmentPlan	22087	shares
CY2018	us-gaap	Stock Issued During Period Shares Dividend Reinvestment Plan StockIssuedDuringPeriodSharesDividendReinvestmentPlan	23493	shares
CY2020	us-gaap	Stock Issued During Period Shares Share Based Compensation StockIssuedDuringPeriodSharesShareBasedCompensation	519863	shares
CY2019	us-gaap	Stock Issued During Period Shares Share Based Compensation StockIssuedDuringPeriodSharesShareBasedCompensation	613678	shares
CY2018	us-gaap	Stock Issued During Period Shares Share Based Compensation StockIssuedDuringPeriodSharesShareBasedCompensation	540337	shares
CY2020	us-gaap	Treasury Stock Shares Acquired TreasuryStockSharesAcquired	104103	shares
CY2019	us-gaap	Treasury Stock Shares Acquired TreasuryStockSharesAcquired	123166	shares
CY2018	us-gaap	Treasury Stock Shares Acquired TreasuryStockSharesAcquired	110398	shares
CY2020Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	59905803	shares
CY2019Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	59461153	shares
CY2018Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	58948554	shares
CY2020	us-gaap	Net Income Loss NetIncomeLoss	246355000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	271623000	USD
CY2018	us-gaap	Net Income Loss NetIncomeLoss	178939000	USD
CY2020	us-gaap	Depreciation Amortization And Accretion Net DepreciationAmortizationAndAccretionNet	59350000	USD
CY2019	us-gaap	Depreciation Amortization And Accretion Net DepreciationAmortizationAndAccretionNet	55205000	USD
CY2018	us-gaap	Depreciation Amortization And Accretion Net DepreciationAmortizationAndAccretionNet	44874000	USD
CY2020	us-gaap	Share Based Compensation ShareBasedCompensation	16227000	USD
CY2019	us-gaap	Share Based Compensation ShareBasedCompensation	19077000	USD
CY2018	us-gaap	Share Based Compensation ShareBasedCompensation	14507000	USD
CY2020	us-gaap	Income Loss From Equity Method Investments Net Of Dividends Or Distributions IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributions	12408000	USD
CY2019	us-gaap	Income Loss From Equity Method Investments Net Of Dividends Or Distributions IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributions	12773000	USD
CY2018	us-gaap	Income Loss From Equity Method Investments Net Of Dividends Or Distributions IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributions	8341000	USD
CY2020	sigi	Proceeds From Equity Investments In Excess Of Current Year Earnings ProceedsFromEquityInvestmentsInExcessOfCurrentYearEarnings	3472000	USD
CY2019	sigi	Proceeds From Equity Investments In Excess Of Current Year Earnings ProceedsFromEquityInvestmentsInExcessOfCurrentYearEarnings	2807000	USD
CY2018	sigi	Proceeds From Equity Investments In Excess Of Current Year Earnings ProceedsFromEquityInvestmentsInExcessOfCurrentYearEarnings	2924000	USD
CY2020	us-gaap	Gain Loss On Investments GainLossOnInvestments	-4217000	USD
CY2019	us-gaap	Gain Loss On Investments GainLossOnInvestments	14422000	USD
CY2018	us-gaap	Gain Loss On Investments GainLossOnInvestments	-54923000	USD
CY2020	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	-22000	USD
CY2019	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	-42000	USD
CY2018	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	-63000	USD
CY2020	sigi	Increase Decrease In Reserves For Losses And Loss Expenses Net Of Reinsurance Recoverables IncreaseDecreaseInReservesForLossesAndLossExpensesNetOfReinsuranceRecoverables	181839000	USD
CY2019	sigi	Increase Decrease In Reserves For Losses And Loss Expenses Net Of Reinsurance Recoverables IncreaseDecreaseInReservesForLossesAndLossExpensesNetOfReinsuranceRecoverables	149232000	USD
CY2018	sigi	Increase Decrease In Reserves For Losses And Loss Expenses Net Of Reinsurance Recoverables IncreaseDecreaseInReservesForLossesAndLossExpensesNetOfReinsuranceRecoverables	168288000	USD
CY2020	sigi	Increase Decrease In Unearned Premiums Net Of Prepaid Reinsurance IncreaseDecreaseInUnearnedPremiumsNetOfPrepaidReinsurance	91278000	USD
CY2019	sigi	Increase Decrease In Unearned Premiums Net Of Prepaid Reinsurance IncreaseDecreaseInUnearnedPremiumsNetOfPrepaidReinsurance	82253000	USD
CY2018	sigi	Increase Decrease In Unearned Premiums Net Of Prepaid Reinsurance IncreaseDecreaseInUnearnedPremiumsNetOfPrepaidReinsurance	78058000	USD
CY2020	us-gaap	Increase Decrease In Income Taxes IncreaseDecreaseInIncomeTaxes	7708000	USD
CY2019	us-gaap	Increase Decrease In Income Taxes IncreaseDecreaseInIncomeTaxes	7721000	USD
CY2018	us-gaap	Increase Decrease In Income Taxes IncreaseDecreaseInIncomeTaxes	2428000	USD
CY2020	us-gaap	Increase Decrease In Premiums Receivable IncreaseDecreaseInPremiumsReceivable	13171000	USD
CY2019	us-gaap	Increase Decrease In Premiums Receivable IncreaseDecreaseInPremiumsReceivable	53383000	USD
CY2018	us-gaap	Increase Decrease In Premiums Receivable IncreaseDecreaseInPremiumsReceivable	23489000	USD
CY2020	us-gaap	Increase Decrease In Deferred Policy Acquisition Costs IncreaseDecreaseInDeferredPolicyAcquisitionCosts	-17392000	USD
CY2019	us-gaap	Increase Decrease In Deferred Policy Acquisition Costs IncreaseDecreaseInDeferredPolicyAcquisitionCosts	-18574000	USD
CY2018	us-gaap	Increase Decrease In Deferred Policy Acquisition Costs IncreaseDecreaseInDeferredPolicyAcquisitionCosts	-17557000	USD
CY2020	us-gaap	Increase Decrease In Interest And Dividends Receivable IncreaseDecreaseInInterestAndDividendsReceivable	158000	USD
CY2019	us-gaap	Increase Decrease In Interest And Dividends Receivable IncreaseDecreaseInInterestAndDividendsReceivable	3226000	USD
CY2018	us-gaap	Increase Decrease In Interest And Dividends Receivable IncreaseDecreaseInInterestAndDividendsReceivable	540000	USD
CY2020	us-gaap	Increase Decrease In Employee Related Liabilities IncreaseDecreaseInEmployeeRelatedLiabilities	-13264000	USD
CY2019	us-gaap	Increase Decrease In Employee Related Liabilities IncreaseDecreaseInEmployeeRelatedLiabilities	-3748000	USD
CY2018	us-gaap	Increase Decrease In Employee Related Liabilities IncreaseDecreaseInEmployeeRelatedLiabilities	-26418000	USD
CY2020	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	27927000	USD
CY2019	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	39337000	USD
CY2018	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	372000	USD
CY2020	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	27897000	USD
CY2020	us-gaap	Proceeds From Maturities Prepayments And Calls Of Held To Maturity Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities	3888000	USD
CY2019	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	34998000	USD
CY2018	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	-13343000	USD
CY2020	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	554045000	USD
CY2019	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	477495000	USD
CY2018	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	454944000	USD
CY2020	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	0	USD
CY2019	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	0	USD
CY2018	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	7150000	USD
CY2020	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	1723818000	USD
CY2019	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	1856125000	USD
CY2018	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	2918203000	USD
CY2020	sigi	Payments To Acquire Commercial Mortgage Loans PaymentsToAcquireCommercialMortgageLoans	-46506000	USD
CY2019	sigi	Payments To Acquire Commercial Mortgage Loans PaymentsToAcquireCommercialMortgageLoans	0	USD
CY2018	sigi	Payments To Acquire Commercial Mortgage Loans PaymentsToAcquireCommercialMortgageLoans	0	USD
CY2020	sigi	Paymentsto Acquire Equity Securities PaymentstoAcquireEquitySecurities	230813000	USD
CY2019	sigi	Paymentsto Acquire Equity Securities PaymentstoAcquireEquitySecurities	46397000	USD
CY2018	sigi	Paymentsto Acquire Equity Securities PaymentstoAcquireEquitySecurities	94344000	USD
CY2020	us-gaap	Payments To Acquire Other Investments PaymentsToAcquireOtherInvestments	79598000	USD
CY2019	us-gaap	Payments To Acquire Other Investments PaymentsToAcquireOtherInvestments	64908000	USD
CY2018	us-gaap	Payments To Acquire Other Investments PaymentsToAcquireOtherInvestments	68578000	USD
CY2020	us-gaap	Payments To Acquire Short Term Investments PaymentsToAcquireShortTermInvestments	5762725000	USD
CY2019	us-gaap	Payments To Acquire Short Term Investments PaymentsToAcquireShortTermInvestments	6087909000	USD
CY2018	us-gaap	Payments To Acquire Short Term Investments PaymentsToAcquireShortTermInvestments	4259734000	USD
CY2020	us-gaap	Proceeds From Sale Of Available For Sale Securities Debt ProceedsFromSaleOfAvailableForSaleSecuritiesDebt	487087000	USD
CY2019	us-gaap	Proceeds From Sale Of Available For Sale Securities Debt ProceedsFromSaleOfAvailableForSaleSecuritiesDebt	594743000	USD
CY2018	us-gaap	Proceeds From Sale Of Available For Sale Securities Debt ProceedsFromSaleOfAvailableForSaleSecuritiesDebt	2030664000	USD
CY2020	us-gaap	Proceeds From Sale And Collection Of Mortgage Notes Receivable ProceedsFromSaleAndCollectionOfMortgageNotesReceivable	201000	USD
CY2019	us-gaap	Proceeds From Sale And Collection Of Mortgage Notes Receivable ProceedsFromSaleAndCollectionOfMortgageNotesReceivable	0	USD
CY2018	us-gaap	Proceeds From Sale And Collection Of Mortgage Notes Receivable ProceedsFromSaleAndCollectionOfMortgageNotesReceivable	0	USD
CY2020	us-gaap	Proceeds From Sale Of Short Term Investments ProceedsFromSaleOfShortTermInvestments	5635463000	USD
CY2019	us-gaap	Proceeds From Sale Of Short Term Investments ProceedsFromSaleOfShortTermInvestments	6129885000	USD
CY2018	us-gaap	Proceeds From Sale Of Short Term Investments ProceedsFromSaleOfShortTermInvestments	4101530000	USD
CY2019	us-gaap	Proceeds From Maturities Prepayments And Calls Of Held To Maturity Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities	16149000	USD
CY2018	us-gaap	Proceeds From Maturities Prepayments And Calls Of Held To Maturity Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities	12106000	USD
CY2020	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	1019132000	USD
CY2019	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	626686000	USD
CY2018	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	638916000	USD
CY2020	sigi	Proceedsfrom Saleof Equity Securities ProceedsfromSaleofEquitySecurities	1320000	USD
CY2019	sigi	Proceedsfrom Saleof Equity Securities ProceedsfromSaleofEquitySecurities	137294000	USD
CY2018	sigi	Proceedsfrom Saleof Equity Securities ProceedsfromSaleofEquitySecurities	113339000	USD
CY2020	us-gaap	Proceeds From Sale Of Equity Method Investments ProceedsFromSaleOfEquityMethodInvestments	5375000	USD
CY2019	us-gaap	Proceeds From Sale Of Equity Method Investments ProceedsFromSaleOfEquityMethodInvestments	17964000	USD
CY2018	us-gaap	Proceeds From Sale Of Equity Method Investments ProceedsFromSaleOfEquityMethodInvestments	3497000	USD
CY2020	us-gaap	Proceeds From Equity Method Investment Dividends Or Distributions Return Of Capital ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	24884000	USD
CY2019	us-gaap	Proceeds From Equity Method Investment Dividends Or Distributions Return Of Capital ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	19972000	USD
CY2018	us-gaap	Proceeds From Equity Method Investment Dividends Or Distributions Return Of Capital ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	28379000	USD
CY2020	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	0	USD
CY2019	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	9000	USD
CY2018	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	0	USD
CY2020	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	22064000	USD
CY2019	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	30986000	USD
CY2018	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	16110000	USD
CY2020	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-688174000	USD
CY2019	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-543623000	USD
CY2018	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-435688000	USD
CY2020	us-gaap	Payments Of Dividends Preferred Stock And Preference Stock PaymentsOfDividendsPreferredStockAndPreferenceStock	0	USD
CY2019	us-gaap	Payments Of Dividends Preferred Stock And Preference Stock PaymentsOfDividendsPreferredStockAndPreferenceStock	0	USD
CY2018	us-gaap	Payments Of Dividends Preferred Stock And Preference Stock PaymentsOfDividendsPreferredStockAndPreferenceStock	0	USD
CY2020	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	54486000	USD
CY2019	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	47675000	USD
CY2018	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	42097000	USD
CY2020	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	7053000	USD
CY2019	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	8164000	USD
CY2018	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	6556000	USD
CY2019	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	57184000	USD
CY2018	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-47047000	USD
CY2020	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans Including Stock Options ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions	8411000	USD
CY2019	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans Including Stock Options ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions	8243000	USD
CY2018	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans Including Stock Options ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions	7252000	USD
CY2020	us-gaap	Proceeds From Issuance Of Preferred Stock And Preference Stock ProceedsFromIssuanceOfPreferredStockAndPreferenceStock	195063000	USD
CY2019	us-gaap	Proceeds From Issuance Of Preferred Stock And Preference Stock ProceedsFromIssuanceOfPreferredStockAndPreferenceStock	0	USD
CY2018	us-gaap	Proceeds From Issuance Of Preferred Stock And Preference Stock ProceedsFromIssuanceOfPreferredStockAndPreferenceStock	0	USD
CY2020	sigi	Borrowings From Subsidiaries BorrowingsFromSubsidiaries	587000000	USD
CY2019	sigi	Borrowings From Subsidiaries BorrowingsFromSubsidiaries	355757000	USD
CY2018	sigi	Borrowings From Subsidiaries BorrowingsFromSubsidiaries	130000000	USD
CY2020	us-gaap	Repayments Of Federal Home Loan Bank Borrowings RepaymentsOfFederalHomeLoanBankBorrowings	587000000	USD
CY2019	us-gaap	Repayments Of Federal Home Loan Bank Borrowings RepaymentsOfFederalHomeLoanBankBorrowings	250000000	USD
CY2018	us-gaap	Repayments Of Federal Home Loan Bank Borrowings RepaymentsOfFederalHomeLoanBankBorrowings	130000000	USD
CY2020	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	550000	USD
CY2019	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	977000	USD
CY2018	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	5646000	USD
CY2020	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	141385000	USD
CY2020	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	7256000	USD
CY2019	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	-8944000	USD
CY2018	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Excluding Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect	-27791000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	7975000	USD
CY2018Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	16919000	USD
CY2017Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	44710000	USD
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	15231000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	7975000	USD
CY2018Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	16919000	USD
CY2020	us-gaap	Nature Of Operations NatureOfOperations	Organization<div><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">Selective Insurance Group, Inc., through its subsidiaries, (collectively referred to as “we,” “us,” or “our”) offers standard commercial, standard personal, and excess and surplus ("E&S") lines property and casualty insurance products. Selective Insurance Group, Inc. (referred to as the “Parent”) was incorporated in New Jersey in 1977 and its corporate headquarters is located in Branchville, New Jersey. The Parent’s common and preferred stock are publicly traded on the NASDAQ Global Select Market under the symbols “SIGI” and "SIGIP," respectively. We have provided a glossary of terms as Exhibit 99.1 to this Form 10-K, which defines certain industry-specific and other terms that are used in this Form 10-K.</span></div><div><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%"> </span></div><div style="margin-bottom:10pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">We classify our business into four reportable segments, which are as follows:</span></div><div style="padding-left:36pt;text-indent:-18pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">•</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%;padding-left:14.5pt">Standard Commercial Lines – comprised of property and casualty insurance products and services provided in the standard marketplace to commercial enterprises, which are typically businesses, non-profit organizations, and local government agencies.</span></div><div style="padding-left:36pt"><span><br/></span></div><div style="padding-left:36pt;text-indent:-18pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">•</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%;padding-left:14.5pt">Standard Personal Lines – comprised of property and casualty insurance products and services, including flood insurance coverage, provided primarily to individuals acquiring coverage in the standard marketplace.</span></div><div style="padding-left:36pt"><span><br/></span></div><div style="padding-left:36pt;text-indent:-18pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">•</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%;padding-left:14.5pt">E&S Lines – comprised of property and casualty insurance products and services provided to customers who have not obtained coverage in the standard marketplace.</span></div><div style="padding-left:36pt"><span><br/></span></div><div style="padding-left:36pt;text-indent:-18pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">•</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%;padding-left:14.5pt">Investments – invests the premiums collected by our insurance operations, as well as amounts generated through our capital management strategies, which may include the issuance of debt and equity securities.</span></div>
CY2020	us-gaap	Use Of Estimates UseOfEstimates	Use of EstimatesThe preparation of our Financial Statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported financial statement balances, as well as the disclosure of contingent assets and liabilities. Actual results could differ from those estimates.
CY2020	us-gaap	Depreciation Depreciation	21500000	USD
CY2019	us-gaap	Depreciation Depreciation	18700000	USD
CY2018	us-gaap	Depreciation Depreciation	19500000	USD
	sigi	Investment Yields Assumed In Premium Deficiency Assessment InvestmentYieldsAssumedInPremiumDeficiencyAssessment	0.030
	sigi	Investment Yields Assumed In Premium Deficiency Assessment InvestmentYieldsAssumedInPremiumDeficiencyAssessment	0.035
	sigi	Investment Yields Assumed In Premium Deficiency Assessment InvestmentYieldsAssumedInPremiumDeficiencyAssessment	0.033
CY2020	sigi	Interest Paid Operatingand Investing Activities InterestPaidOperatingandInvestingActivities	30464000	USD
CY2019	sigi	Interest Paid Operatingand Investing Activities InterestPaidOperatingandInvestingActivities	25089000	USD
CY2018	sigi	Interest Paid Operatingand Investing Activities InterestPaidOperatingandInvestingActivities	23992000	USD
CY2020	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	47000000	USD
CY2019	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	55825000	USD
CY2018	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	29193000	USD
CY2020	us-gaap	Operating Lease Payments OperatingLeasePayments	9498000	USD
CY2019	us-gaap	Operating Lease Payments OperatingLeasePayments	8138000	USD
CY2018	us-gaap	Operating Lease Payments OperatingLeasePayments	0	USD
CY2020	us-gaap	Finance Lease Interest Payment On Liability FinanceLeaseInterestPaymentOnLiability	15000	USD
CY2019	us-gaap	Finance Lease Interest Payment On Liability FinanceLeaseInterestPaymentOnLiability	16000	USD
CY2018	us-gaap	Finance Lease Interest Payment On Liability FinanceLeaseInterestPaymentOnLiability	0	USD
CY2020	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	550000	USD
CY2019	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	977000	USD
CY2018	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	5646000	USD
CY2020	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	324000	USD
CY2019	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	824000	USD
CY2018	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	4119000	USD
CY2020	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	22390000	USD
CY2019	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	13808000	USD
CY2018	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	0	USD
CY2020	us-gaap	Noncash Or Part Noncash Acquisition Fixed Assets Acquired1 NoncashOrPartNoncashAcquisitionFixedAssetsAcquired1	590000	USD
CY2019	us-gaap	Noncash Or Part Noncash Acquisition Fixed Assets Acquired1 NoncashOrPartNoncashAcquisitionFixedAssetsAcquired1	89000	USD
CY2018	us-gaap	Noncash Or Part Noncash Acquisition Fixed Assets Acquired1 NoncashOrPartNoncashAcquisitionFixedAssetsAcquired1	291000	USD
CY2020Q4	us-gaap	Cash Cash	394000	USD
CY2019Q4	us-gaap	Cash Cash	300000	USD
CY2020Q4	us-gaap	Restricted Cash RestrictedCash	14837000	USD
CY2019Q4	us-gaap	Restricted Cash RestrictedCash	7675000	USD
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	15231000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	7975000	USD
CY2020Q4	sigi	Unrealized Gain Loss On Investment Securities UnrealizedGainLossOnInvestmentSecurities	386393000	USD
CY2019Q4	sigi	Unrealized Gain Loss On Investment Securities UnrealizedGainLossOnInvestmentSecurities	215688000	USD
CY2018Q4	sigi	Unrealized Gain Loss On Investment Securities UnrealizedGainLossOnInvestmentSecurities	2391000	USD
CY2020Q4	sigi	Deferred Income Tax Expense Benefit On Investment Securities In Other Comprehensive Income DeferredIncomeTaxExpenseBenefitOnInvestmentSecuritiesInOtherComprehensiveIncome	81142000	USD
CY2019Q4	sigi	Deferred Income Tax Expense Benefit On Investment Securities In Other Comprehensive Income DeferredIncomeTaxExpenseBenefitOnInvestmentSecuritiesInOtherComprehensiveIncome	45294000	USD
CY2018Q4	sigi	Deferred Income Tax Expense Benefit On Investment Securities In Other Comprehensive Income DeferredIncomeTaxExpenseBenefitOnInvestmentSecuritiesInOtherComprehensiveIncome	502000	USD
CY2020Q4	sigi	Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax	305251000	USD
CY2019Q4	sigi	Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax	170394000	USD
CY2018Q4	sigi	Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax	1889000	USD
CY2020	sigi	Total Unrealized Gains Losseson Investment Securities TotalUnrealizedGainsLossesonInvestmentSecurities	134857000	USD
CY2019	sigi	Total Unrealized Gains Losseson Investment Securities TotalUnrealizedGainsLossesonInvestmentSecurities	168505000	USD
CY2018	sigi	Total Unrealized Gains Losseson Investment Securities TotalUnrealizedGainsLossesonInvestmentSecurities	-65881000	USD
CY2020Q4	us-gaap	Available For Sale Securities Amortized Cost AvailableForSaleSecuritiesAmortizedCost	6073517000	USD
CY2020Q4	us-gaap	Debt Securities Available For Sale Allowance For Credit Loss DebtSecuritiesAvailableForSaleAllowanceForCreditLoss	3969000	USD
CY2020Q4	us-gaap	Available For Sale Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	397922000	USD
CY2020Q4	us-gaap	Available For Sale Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	11542000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	6455928000	USD
CY2019Q4	us-gaap	Available For Sale Securities Amortized Cost AvailableForSaleSecuritiesAmortizedCost	5879986000	USD
CY2019Q4	us-gaap	Available For Sale Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	223372000	USD
CY2019Q4	us-gaap	Available For Sale Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	7738000	USD
CY2019Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	6095620000	USD
CY2019Q4	us-gaap	Debt Securities Available For Sale Allowance For Credit Loss DebtSecuritiesAvailableForSaleAllowanceForCreditLoss	0	USD
CY2020	us-gaap	Debt Securities Available For Sale Allowance For Credit Loss Not Previously Recorded DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotPreviouslyRecorded	5042000	USD
CY2020	us-gaap	Debt Securities Available For Sale Allowance For Credit Loss Not To Sell Before Recovery Credit Loss Previously Recorded Expense Reversal DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotToSellBeforeRecoveryCreditLossPreviouslyRecordedExpenseReversal	0	USD
CY2020	us-gaap	Debt Securities Available For Sale Allowance For Credit Loss Securities Sold DebtSecuritiesAvailableForSaleAllowanceForCreditLossSecuritiesSold	-974000	USD
CY2020	us-gaap	Debt Securities Available For Sale Allowance For Credit Loss Sell Before Recovery DebtSecuritiesAvailableForSaleAllowanceForCreditLossSellBeforeRecovery	-99000	USD
CY2020Q4	us-gaap	Debt Securities Available For Sale Allowance For Credit Loss DebtSecuritiesAvailableForSaleAllowanceForCreditLoss	3969000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Maturities Within One Year Fair Value AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue	433241000	USD
CY2020Q4	us-gaap	Held To Maturity Securities Debt Maturities Within One Year Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount	1132000	USD
CY2020Q4	us-gaap	Held To Maturity Securities Debt Maturities Within One Year Fair Value HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue	1149000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Maturities After One Through Five Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue	3639658000	USD
CY2020Q4	us-gaap	Held To Maturity Securities Debt Maturities After One Through Five Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount	15692000	USD
CY2020Q4	us-gaap	Held To Maturity Securities Debt Maturities After One Through Five Years Fair Value HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue	16852000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue	1910480000	USD
CY2020Q4	us-gaap	Held To Maturity Securities Debt Maturities After Five Through Ten Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount	0	USD
CY2020Q4	us-gaap	Held To Maturity Securities Debt Maturities After Five Through Ten Years Fair Value HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue	0	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Maturities After Ten Years Fair Value AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue	472549000	USD
CY2020Q4	us-gaap	Held To Maturity Securities Debt Maturities After Ten Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount	0	USD
CY2020Q4	us-gaap	Held To Maturity Securities Debt Maturities After Ten Years Fair Value HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue	0	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	6455928000	USD
CY2020Q4	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	16824000	USD
CY2020Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	18001000	USD
CY2020	sigi	Schedule Of Other Investment Portfolio By Strategy And Remaining Commitment Amount Associated With Each Strategy Textblock ScheduleOfOtherInvestmentPortfolioByStrategyAndRemainingCommitmentAmountAssociatedWithEachStrategyTextblock	<table style="border-collapse:collapse;display:inline-table;vertical-align:top;width:99.561%"><tr><td style="width:1.0%"/><td style="width:22.835%"/><td style="width:0.1%"/><td style="width:0.1%"/><td style="width:0.534%"/><td style="width:0.1%"/><td style="width:1.0%"/><td style="width:9.619%"/><td style="width:0.1%"/><td style="width:0.1%"/><td style="width:0.534%"/><td style="width:0.1%"/><td style="width:1.0%"/><td style="width:10.500%"/><td style="width:0.1%"/><td style="width:0.1%"/><td style="width:0.534%"/><td style="width:0.1%"/><td style="width:1.0%"/><td style="width:12.409%"/><td style="width:0.1%"/><td style="width:0.1%"/><td style="width:0.534%"/><td style="width:0.1%"/><td style="width:1.0%"/><td style="width:9.619%"/><td style="width:0.1%"/><td style="width:0.1%"/><td style="width:0.534%"/><td style="width:0.1%"/><td style="width:1.0%"/><td style="width:10.500%"/><td style="width:0.1%"/><td style="width:0.1%"/><td style="width:0.534%"/><td style="width:0.1%"/><td style="width:1.0%"/><td style="width:12.414%"/><td style="width:0.1%"/></tr><tr><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">Other Investments</span></td><td colspan="3" style="border-top:1pt solid #000000;padding:0 1pt"/><td colspan="15" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:center;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">December 31, 2020</span></td><td colspan="3" style="border-top:1pt solid #000000;padding:0 1pt"/><td colspan="15" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:center;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">December 31, 2019</span></td></tr><tr><td colspan="3" style="padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">($ in thousands)</span></td><td colspan="3" style="padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:center;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">Carrying<br/>Value</span></td><td colspan="3" style="border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:center;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">Remaining<br/>Commitment</span></td><td colspan="3" style="border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:left;vertical-align:bottom"><div style="text-align:center"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">Maximum</span></div><div style="text-align:center"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">Exposure to Loss</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:5.2pt;font-weight:700;line-height:100%;position:relative;top:-2.8pt;vertical-align:baseline">1</span></div></td><td colspan="3" style="padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:center;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Carrying<br/>Value</span></td><td colspan="3" style="border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:center;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Remaining<br/>Commitment</span></td><td colspan="3" style="border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:left;vertical-align:bottom"><div style="text-align:center"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Maximum<br/>Exposure to Loss</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:5.2pt;font-weight:400;line-height:100%;position:relative;top:-2.8pt;vertical-align:baseline">1</span></div></td></tr><tr><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">Alternative Investments</span></td><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%"> </span></td><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td></tr><tr><td colspan="3" style="padding:2px 1pt 2px 7.75pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Private equity</span></td><td colspan="3" style="padding:0 1pt"/><td style="padding:2px 0 2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">$</span></td><td style="padding:2px 0;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">157,276</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">100,905</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">258,181</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">118,352 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">93,138 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">211,490 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="background-color:#ccffcc;padding:2px 1pt 2px 7.75pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Private credit</span></td><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">54,017</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">98,330</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">152,347</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">42,532 </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">105,340 </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">147,872 </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="padding:2px 1pt 2px 7.75pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Real assets</span></td><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">19,659</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">16,493</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">36,152</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">23,256 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">20,741 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">43,997 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/></tr><tr><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/></tr><tr><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/></tr><tr><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/><td colspan="3" style="display:none"/></tr><tr><td colspan="3" style="background-color:#ccffcc;padding:2px 1pt 2px 12.25pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Total alternative investments</span></td><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">230,952</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">215,728</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">446,680</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">184,140 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">219,219 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">403,359 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Other securities</span></td><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">35,370</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">—</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">35,370</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">32,667 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">— </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">32,667 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="background-color:#ccffcc;padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Total other investments</span></td><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">$</span></td><td style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">266,322</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">215,728</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">482,050</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">216,807 </span></td><td style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">219,219 </span></td><td style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">436,026 </span></td><td style="background-color:#ccffcc;border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr></table>
CY2020Q4	sigi	Maximumexposuretocreditconcentrationriskofthe Companysstockholdersequityotherthancertain U.S.Governmentagencies MaximumexposuretocreditconcentrationriskoftheCompanysstockholdersequityotherthancertainU.S.governmentagencies	0.10
CY2020	us-gaap	Net Investment Income NetInvestmentIncome	227107000	USD
CY2019	us-gaap	Net Investment Income NetInvestmentIncome	222543000	USD
CY2018	us-gaap	Net Investment Income NetInvestmentIncome	195336000	USD
CY2020	sigi	Debt And Equity Securities Realized Gains DebtAndEquitySecuritiesRealizedGains	18893000	USD
CY2019	sigi	Debt And Equity Securities Realized Gains DebtAndEquitySecuritiesRealizedGains	31910000	USD
CY2018	sigi	Debt And Equity Securities Realized Gains DebtAndEquitySecuritiesRealizedGains	28672000	USD
CY2020	sigi	Debt And Equity Securities Realized Losses DebtAndEquitySecuritiesRealizedLosses	9745000	USD
CY2019	sigi	Debt And Equity Securities Realized Losses DebtAndEquitySecuritiesRealizedLosses	5195000	USD
CY2018	sigi	Debt And Equity Securities Realized Losses DebtAndEquitySecuritiesRealizedLosses	47647000	USD
CY2020	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	9148000	USD
CY2019	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	26715000	USD
CY2018	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	-18975000	USD
CY2020	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	7939000	USD
CY2019	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	-8649000	USD
CY2018	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	-29369000	USD
CY2020	sigi	Losses On Investment Securities For Which We Have The Intent To Sell LossesOnInvestmentSecuritiesForWhichWeHaveTheIntentToSell	-16266000	USD
CY2019	us-gaap	Impairment Of Investments ImpairmentOfInvestments	-3644000	USD
CY2018	us-gaap	Impairment Of Investments ImpairmentOfInvestments	-6579000	USD
CY2020	us-gaap	Gain Loss On Investments GainLossOnInvestments	-4217000	USD
CY2019	us-gaap	Gain Loss On Investments GainLossOnInvestments	14422000	USD
CY2018	us-gaap	Gain Loss On Investments GainLossOnInvestments	-54923000	USD
CY2020	us-gaap	Unrealized Gain Loss On Investments UnrealizedGainLossOnInvestments	7936000	USD
CY2019	us-gaap	Unrealized Gain Loss On Investments UnrealizedGainLossOnInvestments	1219000	USD
CY2018	us-gaap	Unrealized Gain Loss On Investments UnrealizedGainLossOnInvestments	-3098000	USD
CY2020	sigi	Unrealized Gain Losson Equity Securities Sold UnrealizedGainLossonEquitySecuritiesSold	3000	USD
CY2019	sigi	Unrealized Gain Losson Equity Securities Sold UnrealizedGainLossonEquitySecuritiesSold	-9868000	USD
CY2018	sigi	Unrealized Gain Losson Equity Securities Sold UnrealizedGainLossonEquitySecuritiesSold	-26271000	USD
CY2020	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	7939000	USD
CY2019	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	-8649000	USD
CY2018	us-gaap	Equity Securities Fv Ni Unrealized Gain Loss EquitySecuritiesFvNiUnrealizedGainLoss	-29369000	USD
CY2020	us-gaap	Proceeds From Sale Of Available For Sale Securities Debt ProceedsFromSaleOfAvailableForSaleSecuritiesDebt	487100000	USD
CY2019	us-gaap	Proceeds From Sale Of Available For Sale Securities Debt ProceedsFromSaleOfAvailableForSaleSecuritiesDebt	594700000	USD
CY2018	us-gaap	Proceeds From Sale Of Available For Sale Securities Debt ProceedsFromSaleOfAvailableForSaleSecuritiesDebt	2030700000	USD
CY2020	sigi	Proceedsfrom Saleof Equity Securities ProceedsfromSaleofEquitySecurities	1300000	USD
CY2019	sigi	Proceedsfrom Saleof Equity Securities ProceedsfromSaleofEquitySecurities	137300000	USD
CY2018	sigi	Proceedsfrom Saleof Equity Securities ProceedsfromSaleofEquitySecurities	113300000	USD
CY2020	sigi	Losses On Investment Securities For Which We Have The Intent To Sell LossesOnInvestmentSecuritiesForWhichWeHaveTheIntentToSell	-16300000	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	302988000	USD
CY2020	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	56633000	USD
CY2020	us-gaap	Net Income Loss NetIncomeLoss	246355000	USD
CY2020	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	168487000	USD
CY2020	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	35383000	USD
CY2020	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	133104000	USD
CY2020	sigi	Non Credit Portionof Other Than Temporary Impairments Recognizedin Other Comprehensive Income Gross NonCreditPortionofOtherThanTemporaryImpairmentsRecognizedinOtherComprehensiveIncomeGross	-8176000	USD
CY2020	sigi	Non Credit Portionof Other Than Temporary Impairments Recognizedin Other Comprehensive Income Tax NonCreditPortionofOtherThanTemporaryImpairmentsRecognizedinOtherComprehensiveIncomeTax	-1717000	USD
CY2020	sigi	Non Credit Portionof Other Than Temporary Impairments Recognizedin Other Comprehensive Income NonCreditPortionofOtherThanTemporaryImpairmentsRecognizedinOtherComprehensiveIncome	-6459000	USD
CY2020	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Before Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax	-24000	USD
CY2020	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax	-5000	USD
CY2020	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Netof Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax	-19000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	-5376000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax	-1129000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	-4247000	USD
CY2020	sigi	Debt Securities Availableforsale Allowancefor Credit Loss Gross DebtSecuritiesAvailableforsaleAllowanceforCreditLossGross	5042000	USD
CY2020	sigi	Debt Securities Availableforsale Allowancefor Credit Loss Tax DebtSecuritiesAvailableforsaleAllowanceforCreditLossTax	1058000	USD
CY2020	sigi	Debt Securities Availableforsale Allowancefor Credit Loss Net DebtSecuritiesAvailableforsaleAllowanceforCreditLossNet	3984000	USD
CY2020	sigi	Total Unrealized Gains Losses On Investment Securities Gross TotalUnrealizedGainsLossesOnInvestmentSecuritiesGross	170705000	USD
CY2020	sigi	Total Unrealized Gains Losses On Investment Securities Tax TotalUnrealizedGainsLossesOnInvestmentSecuritiesTax	35848000	USD
CY2020	sigi	Total Unrealized Gains Losseson Investment Securities TotalUnrealizedGainsLossesonInvestmentSecurities	134857000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Net Unamortized Gain Loss Arising During Period Before Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax	1515000	USD
CY2020	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodTax	318000	USD
CY2020	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Net Of Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax	1197000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax	-3015000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax	-633000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax	-2382000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Before Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax	-4530000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax	-951000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	-3579000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	175235000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	36799000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	138436000	USD
CY2020	sigi	Comprehensive Income Loss Before Tax ComprehensiveIncomeLossBeforeTax	478223000	USD
CY2020	sigi	Comprehensive Income Loss Tax ComprehensiveIncomeLossTax	93432000	USD
CY2020	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	384791000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	336390000	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	64767000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	271623000	USD
CY2019	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	212683000	USD
CY2019	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	44662000	USD
CY2019	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	168021000	USD
CY2019	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Before Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax	-58000	USD
CY2019	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax	-12000	USD
CY2019	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Netof Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax	-46000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	-671000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax	-141000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	-530000	USD
CY2019	sigi	Total Unrealized Gains Losses On Investment Securities Gross TotalUnrealizedGainsLossesOnInvestmentSecuritiesGross	213296000	USD
CY2019	sigi	Total Unrealized Gains Losses On Investment Securities Tax TotalUnrealizedGainsLossesOnInvestmentSecuritiesTax	44791000	USD
CY2019	sigi	Total Unrealized Gains Losseson Investment Securities TotalUnrealizedGainsLossesonInvestmentSecurities	168505000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Net Unamortized Gain Loss Arising During Period Before Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax	-13795000	USD
CY2019	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodTax	-2897000	USD
CY2019	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Net Of Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax	-10898000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax	-2657000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax	-558000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax	-2099000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Before Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax	11138000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax	2339000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	8799000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	202158000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	42452000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	159706000	USD
CY2019	sigi	Comprehensive Income Loss Before Tax ComprehensiveIncomeLossBeforeTax	538548000	USD
CY2019	sigi	Comprehensive Income Loss Tax ComprehensiveIncomeLossTax	107219000	USD
CY2019	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	431329000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	211721000	USD
CY2018	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	32782000	USD
CY2018	us-gaap	Net Income Loss NetIncomeLoss	178939000	USD
CY2018	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	-123145000	USD
CY2018	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	-25861000	USD
CY2018	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	-97284000	USD
CY2018	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Before Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleBeforeTax	110000	USD
CY2018	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleTax	23000	USD
CY2018	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Netof Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax	87000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	-39641000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax	-8325000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	-31316000	USD
CY2018	sigi	Total Unrealized Gains Losses On Investment Securities Gross TotalUnrealizedGainsLossesOnInvestmentSecuritiesGross	-83394000	USD
CY2018	sigi	Total Unrealized Gains Losses On Investment Securities Tax TotalUnrealizedGainsLossesOnInvestmentSecuritiesTax	-17513000	USD
CY2018	sigi	Total Unrealized Gains Losseson Investment Securities TotalUnrealizedGainsLossesonInvestmentSecurities	-65881000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Net Unamortized Gain Loss Arising During Period Before Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax	-11273000	USD
CY2018	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodTax	-2367000	USD
CY2018	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Net Of Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax	-8906000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax	-2127000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax	-447000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci Pension And Other Postretirement Benefit Plans For Net Gain Loss Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax	-1680000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Before Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax	9146000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax	1920000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	7226000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-92540000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-19433000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-73107000	USD
CY2018	sigi	Comprehensive Income Loss Before Tax ComprehensiveIncomeLossBeforeTax	119181000	USD
CY2018	sigi	Comprehensive Income Loss Tax ComprehensiveIncomeLossTax	13349000	USD
CY2018	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	105832000	USD
CY2019	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Netof Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax	-46000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	8799000	USD
CY2020	sigi	Amortization Of Net Unrealized Gains Losses On Held To Maturity Transferred From Available For Sale Netof Tax AmortizationOfNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax	-19000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	-3579000	USD
CY2020Q4	sigi	Net Asset Value NetAssetValue	46800000	USD
CY2019Q4	sigi	Net Asset Value NetAssetValue	37800000	USD
CY2020Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	18001000	USD
CY2020Q4	sigi	Commercial Mortgage Loan Fair Value CommercialMortgageLoanFairValue	47289000	USD
CY2019Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	21975000	USD
CY2019Q4	us-gaap	Premiums Receivable Allowance For Doubtful Accounts PremiumsReceivableAllowanceForDoubtfulAccounts	6400000	USD
CY2020Q4	us-gaap	Premiums Receivable Allowance For Doubtful Accounts PremiumsReceivableAllowanceForDoubtfulAccounts	21000000	USD
CY2020	sigi	Premiums Receivable Allowance Net Of Writeoffs PremiumsReceivableAllowanceNetOfWriteoffs	13500000	USD
CY2020	sigi	Terrorism Risk Insurance Program Companys Estimated Deductible TerrorismRiskInsuranceProgramCompanysEstimatedDeductible	369000000	USD
CY2020	sigi	Terrorism Risk Insurance Program Reimbursed Percentage Maximum TerrorismRiskInsuranceProgramReimbursedPercentageMaximum	0.80
CY2020	sigi	Terrorism Risk Insurance Program Annual Cap TerrorismRiskInsuranceProgramAnnualCap	100000000000	USD
CY2020Q4	sigi	Terrorism Risk Insurance Program Insurer Retainer TerrorismRiskInsuranceProgramInsurerRetainer	0.20
CY2020Q4	us-gaap	Reinsurance Recoverable Credit Quality Indicator Date Updated ReinsuranceRecoverableCreditQualityIndicatorDateUpdated	2020-12-31
CY2020Q4	us-gaap	Reinsurance Recoverables On Unpaid Losses Gross ReinsuranceRecoverablesOnUnpaidLossesGross	589269000	USD
CY2020Q4	us-gaap	Reinsurance Recoverables Allowance ReinsuranceRecoverablesAllowance	1777000	USD
CY2020Q4	us-gaap	Reinsurance Recoverables ReinsuranceRecoverables	587492000	USD
CY2019Q4	us-gaap	Reinsurance Recoverables Allowance ReinsuranceRecoverablesAllowance	4400000	USD
CY2020	us-gaap	Reinsurance Recoverable Credit Loss Expense Reversal ReinsuranceRecoverableCreditLossExpenseReversal	280000	USD
CY2020	us-gaap	Reinsurance Loss On Uncollectible Accounts In Period Amount ReinsuranceLossOnUncollectibleAccountsInPeriodAmount	0	USD
CY2020	us-gaap	Reinsurance Recoverable Allowance For Credit Loss Recovery ReinsuranceRecoverableAllowanceForCreditLossRecovery	0	USD
CY2020Q4	us-gaap	Reinsurance Recoverables Allowance ReinsuranceRecoverablesAllowance	1777000	USD
CY2020Q4	us-gaap	Reinsurance Recoverables ReinsuranceRecoverables	587492000	USD
CY2019Q4	us-gaap	Reinsurance Recoverables ReinsuranceRecoverables	573235000	USD
CY2020Q4	us-gaap	Prepaid Reinsurance Premiums PrepaidReinsurancePremiums	170531000	USD
CY2019Q4	us-gaap	Prepaid Reinsurance Premiums PrepaidReinsurancePremiums	166705000	USD
CY2020Q4	sigi	Remaining Reinsurance Balances RemainingReinsuranceBalances	758023000	USD
CY2019Q4	sigi	Remaining Reinsurance Balances RemainingReinsuranceBalances	739940000	USD
CY2020Q4	sigi	Collateral Of Reinsurance Recoverables CollateralOfReinsuranceRecoverables	130169000	USD
CY2019Q4	sigi	Collateral Of Reinsurance Recoverables CollateralOfReinsuranceRecoverables	110549000	USD
CY2020	us-gaap	Direct Premiums Written DirectPremiumsWritten	3204512000	USD
CY2019	us-gaap	Direct Premiums Written DirectPremiumsWritten	3084451000	USD
CY2018	us-gaap	Direct Premiums Written DirectPremiumsWritten	2890633000	USD
CY2020	us-gaap	Assumed Premiums Written AssumedPremiumsWritten	24288000	USD
CY2019	us-gaap	Assumed Premiums Written AssumedPremiumsWritten	24339000	USD
CY2018	us-gaap	Assumed Premiums Written AssumedPremiumsWritten	26250000	USD
CY2020	us-gaap	Ceded Premiums Written CededPremiumsWritten	455708000	USD
CY2019	us-gaap	Ceded Premiums Written CededPremiumsWritten	429366000	USD
CY2018	us-gaap	Ceded Premiums Written CededPremiumsWritten	402597000	USD
CY2020	us-gaap	Premiums Written Net PremiumsWrittenNet	2773092000	USD
CY2019	us-gaap	Premiums Written Net PremiumsWrittenNet	2679424000	USD
CY2018	us-gaap	Premiums Written Net PremiumsWrittenNet	2514286000	USD
CY2020	us-gaap	Direct Premiums Earned DirectPremiumsEarned	3108687000	USD
CY2019	us-gaap	Direct Premiums Earned DirectPremiumsEarned	2993157000	USD
CY2018	us-gaap	Direct Premiums Earned DirectPremiumsEarned	2808764000	USD
CY2020	us-gaap	Assumed Premiums Earned AssumedPremiumsEarned	25010000	USD
CY2019	us-gaap	Assumed Premiums Earned AssumedPremiumsEarned	24399000	USD
CY2018	us-gaap	Assumed Premiums Earned AssumedPremiumsEarned	25831000	USD
CY2020	us-gaap	Ceded Premiums Earned CededPremiumsEarned	451883000	USD
CY2019	us-gaap	Ceded Premiums Earned CededPremiumsEarned	420385000	USD
CY2018	us-gaap	Ceded Premiums Earned CededPremiumsEarned	398366000	USD
CY2020	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	2681814000	USD
CY2019	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	2597171000	USD
CY2018	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	2436229000	USD
CY2020	us-gaap	Policyholder Benefits And Claims Incurred Gross PolicyholderBenefitsAndClaimsIncurredGross	1822034000	USD
CY2020	sigi	Percentage Of Unpaid Nfip Claims Ceded By Federal Government PercentageOfUnpaidNfipClaimsCededByFederalGovernment	1
CY2019	us-gaap	Policyholder Benefits And Claims Incurred Gross PolicyholderBenefitsAndClaimsIncurredGross	1714880000	USD
CY2018	us-gaap	Policyholder Benefits And Claims Incurred Gross PolicyholderBenefitsAndClaimsIncurredGross	1706951000	USD
CY2020	us-gaap	Reinsurance Effect On Claims And Benefits Incurred Amount Assumed ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed	17201000	USD
CY2019	us-gaap	Reinsurance Effect On Claims And Benefits Incurred Amount Assumed ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed	22879000	USD
CY2018	us-gaap	Reinsurance Effect On Claims And Benefits Incurred Amount Assumed ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed	21469000	USD
CY2020	us-gaap	Reinsurance Effect On Claims And Benefits Incurred Amount Ceded ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded	203412000	USD
CY2019	us-gaap	Reinsurance Effect On Claims And Benefits Incurred Amount Ceded ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded	186268000	USD
CY2018	us-gaap	Reinsurance Effect On Claims And Benefits Incurred Amount Ceded ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded	230286000	USD
CY2020	us-gaap	Incurred Claims Property Casualty And Liability IncurredClaimsPropertyCasualtyAndLiability	1635823000	USD
CY2019	us-gaap	Incurred Claims Property Casualty And Liability IncurredClaimsPropertyCasualtyAndLiability	1551491000	USD
CY2018	us-gaap	Incurred Claims Property Casualty And Liability IncurredClaimsPropertyCasualtyAndLiability	1498134000	USD
CY2020	sigi	Increase In Dpw From Prior Year IncreaseInDPWFromPriorYear	0.04
CY2019	sigi	Increase In Dpw From Prior Year IncreaseInDPWFromPriorYear	0.07
CY2020	sigi	Audit And Endorsement Premium Decrease AuditAndEndorsementPremiumDecrease	82500000	USD
CY2020Q2	sigi	Policyholder Premium Credit PolicyholderPremiumCredit	0.15
CY2020	us-gaap	Loss From Catastrophes LossFromCatastrophes	215400000	USD
CY2019	us-gaap	Loss From Catastrophes LossFromCatastrophes	81000000.0	USD
CY2020	us-gaap	Schedule Of Liability For Unpaid Claims And Claims Adjustment Expense ScheduleOfLiabilityForUnpaidClaimsAndClaimsAdjustmentExpense	<table style="border-collapse:collapse;display:inline-table;vertical-align:top;width:100.000%"><tr><td style="width:1.0%"/><td style="width:61.619%"/><td style="width:0.1%"/><td style="width:0.1%"/><td style="width:0.530%"/><td style="width:0.1%"/><td style="width:1.0%"/><td style="width:10.595%"/><td style="width:0.1%"/><td style="width:0.1%"/><td style="width:0.530%"/><td style="width:0.1%"/><td style="width:1.0%"/><td style="width:10.595%"/><td style="width:0.1%"/><td style="width:0.1%"/><td style="width:0.530%"/><td style="width:0.1%"/><td style="width:1.0%"/><td style="width:10.601%"/><td style="width:0.1%"/></tr><tr><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">($ in thousands)</span></td><td colspan="3" style="border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:center;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">2020</span></td><td colspan="3" style="border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:center;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">2019</span></td><td colspan="3" style="border-top:1pt solid #000000;padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:center;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">2018</span></td></tr><tr><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Gross reserves for loss and loss expense, at beginning of year</span></td><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">$</span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">4,067,163</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">3,893,868 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">3,771,240 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="padding:2px 1pt;text-align:left;vertical-align:bottom"><div><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:5.2pt;font-weight:400;line-height:100%;position:relative;top:-2.8pt;vertical-align:baseline">1</span></div></td><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">547,066</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">537,388 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">585,855 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="background-color:#ccffcc;padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Net reserves for loss and loss expense, at beginning of year</span></td><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">3,520,097</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">3,356,480 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">3,185,385 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Incurred loss and loss expense for claims occurring in the:</span></td><td colspan="3" style="padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%"> </span></td><td colspan="3" style="padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td colspan="3" style="padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td></tr><tr><td colspan="3" style="background-color:#ccffcc;padding:2px 1pt 2px 7.75pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Current year</span></td><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">1,708,755</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">1,601,780 </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">1,527,997 </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="padding:2px 1pt 2px 7.75pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Prior years</span></td><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">(72,932)</span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">(50,289)</span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">(29,863)</span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="background-color:#ccffcc;padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Total incurred loss and loss expense</span></td><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">1,635,823</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">1,551,491 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">1,498,134 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Paid loss and loss expense for claims occurring in the:</span></td><td colspan="3" style="padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%"> </span></td><td colspan="3" style="padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td colspan="3" style="padding:0 1pt"/><td colspan="3" style="border-top:1pt solid #000000;padding:2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td></tr><tr><td colspan="3" style="background-color:#ccffcc;padding:2px 1pt 2px 7.75pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Current year</span></td><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">642,586</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">579,527 </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">573,718 </span></td><td style="background-color:#ccffcc;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="padding:2px 1pt 2px 7.75pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Prior years</span></td><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">807,248</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">805,443 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">753,321 </span></td><td style="padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="background-color:#ccffcc;padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Total paid loss and loss expense</span></td><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">1,449,834</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">1,384,970 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#ccffcc;padding:0 1pt"/><td colspan="2" style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">1,327,039 </span></td><td style="background-color:#ccffcc;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Net reserves for loss and loss expense, at end of year</span></td><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">3,706,086</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">3,523,001 </span></td><td style="border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">3,356,480 </span></td><td style="border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="background-color:#d0ffd0;padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Add: Reinsurance recoverable on unpaid loss and loss expense, at end of year</span></td><td colspan="3" style="background-color:#d0ffd0;padding:0 1pt"/><td colspan="2" style="background-color:#d0ffd0;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">554,269</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="background-color:#d0ffd0;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#d0ffd0;padding:0 1pt"/><td colspan="2" style="background-color:#d0ffd0;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">544,162 </span></td><td style="background-color:#d0ffd0;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="background-color:#d0ffd0;padding:0 1pt"/><td colspan="2" style="background-color:#d0ffd0;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">537,388 </span></td><td style="background-color:#d0ffd0;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr><tr><td colspan="3" style="padding:2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">Gross reserves for loss and loss expense at end of year</span></td><td colspan="3" style="padding:0 1pt"/><td style="border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:left;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">$</span></td><td style="border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:700;line-height:100%">4,260,355</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%"> </span></td><td style="border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">4,067,163 </span></td><td style="border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/><td colspan="3" style="padding:0 1pt"/><td colspan="2" style="border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 0 2px 1pt;text-align:right;vertical-align:bottom"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:8pt;font-weight:400;line-height:100%">3,893,868 </span></td><td style="border-bottom:3pt double #000000;border-top:1pt solid #000000;padding:2px 1pt 2px 0;text-align:right;vertical-align:bottom"/></tr></table>
CY2019Q4	us-gaap	Liability For Claims And Claims Adjustment Expense LiabilityForClaimsAndClaimsAdjustmentExpense	4067163000	USD
CY2018Q4	us-gaap	Liability For Claims And Claims Adjustment Expense LiabilityForClaimsAndClaimsAdjustmentExpense	3893868000	USD
CY2017Q4	us-gaap	Liability For Claims And Claims Adjustment Expense LiabilityForClaimsAndClaimsAdjustmentExpense	3771240000	USD
CY2018Q4	us-gaap	Reinsurance Recoverable For Unpaid Claims And Claims Adjustments ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments	537388000	USD
CY2017Q4	us-gaap	Reinsurance Recoverable For Unpaid Claims And Claims Adjustments ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments	585855000	USD
CY2018Q4	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Net LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet	3356480000	USD
CY2017Q4	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Net LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet	3185385000	USD
CY2020	us-gaap	Supplemental Information For Property Casualty Insurance Underwriters Current Year Claims And Claims Adjustment Expense SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense	1708755000	USD
CY2019	us-gaap	Supplemental Information For Property Casualty Insurance Underwriters Current Year Claims And Claims Adjustment Expense SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense	1601780000	USD
CY2018	us-gaap	Supplemental Information For Property Casualty Insurance Underwriters Current Year Claims And Claims Adjustment Expense SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense	1527997000	USD
CY2020	us-gaap	Supplemental Information For Property Casualty Insurance Underwriters Prior Year Claims And Claims Adjustment Expense SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense	72932000	USD
CY2019	us-gaap	Supplemental Information For Property Casualty Insurance Underwriters Prior Year Claims And Claims Adjustment Expense SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense	50289000	USD
CY2018	us-gaap	Supplemental Information For Property Casualty Insurance Underwriters Prior Year Claims And Claims Adjustment Expense SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense	29863000	USD
CY2020	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims1 LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1	1635823000	USD
CY2019	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims1 LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1	1551491000	USD
CY2018	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims1 LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1	1498134000	USD
CY2020	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Claims Paid Current Year1 LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1	642586000	USD
CY2019	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Claims Paid Current Year1 LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1	579527000	USD
CY2018	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Claims Paid Current Year1 LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1	573718000	USD
CY2020	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Claims Paid Prior Years1 LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1	807248000	USD
CY2019	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Claims Paid Prior Years1 LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1	805443000	USD
CY2018	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Claims Paid Prior Years1 LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1	753321000	USD
CY2020	us-gaap	Payments For Losses And Loss Adjustment Expense PaymentsForLossesAndLossAdjustmentExpense	1449834000	USD
CY2019	us-gaap	Payments For Losses And Loss Adjustment Expense PaymentsForLossesAndLossAdjustmentExpense	1384970000	USD
CY2018	us-gaap	Payments For Losses And Loss Adjustment Expense PaymentsForLossesAndLossAdjustmentExpense	1327039000	USD
CY2020Q4	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Net LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet	3706086000	USD
CY2019Q4	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Net LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet	3523001000	USD
CY2018Q4	us-gaap	Liability For Unpaid Claims And Claims Adjustment Expense Net LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet	3356480000	USD
CY2020Q4	us-gaap	Reinsurance Recoverable For Unpaid Claims And Claims Adjustments ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments	554269000	USD
CY2019Q4	us-gaap	Reinsurance Recoverable For Unpaid Claims And Claims Adjustments ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments	544162000	USD
CY2018	us-gaap	Increase Decrease In Liability For Claims And Claims Adjustment Expense Reserve IncreaseDecreaseInLiabilityForClaimsAndClaimsAdjustmentExpenseReserve	171100000	USD
CY2020Q4	us-gaap	Salvage And Subrogation Recoveries Value SalvageAndSubrogationRecoveriesValue	80900000	USD
CY2019Q4	us-gaap	Salvage And Subrogation Recoveries Value SalvageAndSubrogationRecoveriesValue	76700000	USD
CY2018Q4	us-gaap	Salvage And Subrogation Recoveries Value SalvageAndSubrogationRecoveriesValue	67700000	USD
CY2018Q4	us-gaap	Reinsurance Recoverable For Unpaid Claims And Claims Adjustments ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments	537388000	USD
CY2020Q4	us-gaap	Liability For Claims And Claims Adjustment Expense LiabilityForClaimsAndClaimsAdjustmentExpense	4260355000	USD
CY2019Q4	us-gaap	Liability For Claims And Claims Adjustment Expense LiabilityForClaimsAndClaimsAdjustmentExpense	4067163000	USD
CY2018Q4	us-gaap	Liability For Claims And Claims Adjustment Expense LiabilityForClaimsAndClaimsAdjustmentExpense	3893868000	USD
CY2020	us-gaap	Increase Decrease In Liability For Claims And Claims Adjustment Expense Reserve IncreaseDecreaseInLiabilityForClaimsAndClaimsAdjustmentExpenseReserve	183100000	USD
CY2019	us-gaap	Increase Decrease In Liability For Claims And Claims Adjustment Expense Reserve IncreaseDecreaseInLiabilityForClaimsAndClaimsAdjustmentExpenseReserve	166500000	USD
CY2020Q4	us-gaap	Liability For Asbestos And Environmental Claims Gross LiabilityForAsbestosAndEnvironmentalClaimsGross	28500000	USD
CY2020Q4	us-gaap	Liability For Asbestos And Environmental Claims Net LiabilityForAsbestosAndEnvironmentalClaimsNet	21400000	USD
CY2018Q4	us-gaap	Liability For Asbestos And Environmental Claims Gross LiabilityForAsbestosAndEnvironmentalClaimsGross	30020000	USD
CY2017Q4	us-gaap	Liability For Asbestos And Environmental Claims Gross LiabilityForAsbestosAndEnvironmentalClaimsGross	28415000	USD
CY2018Q4	us-gaap	Liability For Asbestos And Environmental Claims Net LiabilityForAsbestosAndEnvironmentalClaimsNet	22783000	USD
CY2017Q4	us-gaap	Liability For Asbestos And Environmental Claims Net LiabilityForAsbestosAndEnvironmentalClaimsNet	21212000	USD
CY2020	sigi	Liability For Asbestos And Environmental Claims Gross Incurred Loss Sigi LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLossSIGI	-127000	USD
CY2020	sigi	Liability For Asbestos And Environmental Claims Net Incurred Loss Sigi LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLossSIGI	-154000	USD
CY2019Q4	us-gaap	Liability For Asbestos And Environmental Claims Gross LiabilityForAsbestosAndEnvironmentalClaimsGross	28701000	USD
CY2019Q4	us-gaap	Liability For Asbestos And Environmental Claims Net LiabilityForAsbestosAndEnvironmentalClaimsNet	21589000	USD
CY2019	sigi	Liability For Asbestos And Environmental Claims Gross Incurred Loss Sigi LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLossSIGI	348000	USD
CY2019	sigi	Liability For Asbestos And Environmental Claims Net Incurred Loss Sigi LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLossSIGI	234000	USD
CY2018	sigi	Liability For Asbestos And Environmental Claims Gross Incurred Loss Sigi LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLossSIGI	3059000	USD
CY2018	sigi	Liability For Asbestos And Environmental Claims Net Incurred Loss Sigi LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLossSIGI	2877000	USD
CY2020	sigi	Liability For Asbestos And Environmental Claims Gross Payment For Claims Sigi LiabilityForAsbestosAndEnvironmentalClaimsGrossPaymentForClaimsSIGI	-44000	USD
CY2020	sigi	Liability For Asbestos And Environmental Claims Net Payment For Claims Sigi LiabilityForAsbestosAndEnvironmentalClaimsNetPaymentForClaimsSIGI	-14000	USD
CY2019	sigi	Liability For Asbestos And Environmental Claims Gross Payment For Claims Sigi LiabilityForAsbestosAndEnvironmentalClaimsGrossPaymentForClaimsSIGI	-1667000	USD
CY2019	sigi	Liability For Asbestos And Environmental Claims Net Payment For Claims Sigi LiabilityForAsbestosAndEnvironmentalClaimsNetPaymentForClaimsSIGI	-1428000	USD
CY2018	sigi	Liability For Asbestos And Environmental Claims Gross Payment For Claims Sigi LiabilityForAsbestosAndEnvironmentalClaimsGrossPaymentForClaimsSIGI	-1454000	USD
CY2018	sigi	Liability For Asbestos And Environmental Claims Net Payment For Claims Sigi LiabilityForAsbestosAndEnvironmentalClaimsNetPaymentForClaimsSIGI	-1306000	USD
CY2020Q4	us-gaap	Liability For Asbestos And Environmental Claims Gross LiabilityForAsbestosAndEnvironmentalClaimsGross	28530000	USD
CY2020Q4	us-gaap	Liability For Asbestos And Environmental Claims Net LiabilityForAsbestosAndEnvironmentalClaimsNet	21421000	USD
CY2019Q4	us-gaap	Liability For Asbestos And Environmental Claims Gross LiabilityForAsbestosAndEnvironmentalClaimsGross	28701000	USD
CY2019Q4	us-gaap	Liability For Asbestos And Environmental Claims Net LiabilityForAsbestosAndEnvironmentalClaimsNet	21589000	USD
CY2018Q4	us-gaap	Liability For Asbestos And Environmental Claims Gross LiabilityForAsbestosAndEnvironmentalClaimsGross	30020000	USD
CY2018Q4	us-gaap	Liability For Asbestos And Environmental Claims Net LiabilityForAsbestosAndEnvironmentalClaimsNet	22783000	USD
CY2020Q4	us-gaap	Shortduration Insurance Contracts Incurred Claims And Allocated Claim Adjustment Expense Net ShortdurationInsuranceContractsIncurredClaimsAndAllocatedClaimAdjustmentExpenseNet	12185164000	USD
CY2020Q4	us-gaap	Shortduration Insurance Contracts Cumulative Paid Claims And Allocated Claim Adjustment Expense Net ShortdurationInsuranceContractsCumulativePaidClaimsAndAllocatedClaimAdjustmentExpenseNet	8973347000	USD
CY2020Q4	us-gaap	Shortduration Insurance Contracts Liability For Unpaid Claims And Allocated Claim Adjustment Expense Net ShortdurationInsuranceContractsLiabilityForUnpaidClaimsAndAllocatedClaimAdjustmentExpenseNet	3576212000	USD
CY2020Q4	us-gaap	Shortduration Insurance Contracts Liability For Unpaid Claims And Allocated Claim Adjustment Expense Net ShortdurationInsuranceContractsLiabilityForUnpaidClaimsAndAllocatedClaimAdjustmentExpenseNet	3576212000	USD
CY2020Q4	us-gaap	Reinsurance Recoverable For Unpaid Claims And Claims Adjustments ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments	554269000	USD
CY2020Q4	us-gaap	Shortduration Insurance Contracts Liability For Unpaid Claims And Claims Adjustment Expense Accumulated Unallocated Claim Adjustment Expense ShortdurationInsuranceContractsLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAccumulatedUnallocatedClaimAdjustmentExpense	129874000	USD
CY2020Q4	us-gaap	Liability For Claims And Claims Adjustment Expense LiabilityForClaimsAndClaimsAdjustmentExpense	4260355000	USD
CY2020Q4	us-gaap	Deferred Finance Costs Net DeferredFinanceCostsNet	3419000	USD
CY2020Q4	us-gaap	Debt Instrument Unamortized Discount DebtInstrumentUnamortizedDiscount	6346000	USD
CY2020Q4	us-gaap	Long Term Debt LongTermDebt	550743000	USD
CY2019Q4	us-gaap	Long Term Debt LongTermDebt	550597000	USD
CY2020Q4	sigi	Line Of Credit Consolidated Net Worth Requirement LineOfCreditConsolidatedNetWorthRequirement	1600000000	USD
CY2020Q4	sigi	Consolidated Net Worth Actual ConsolidatedNetWorthActual	2500000000	USD
CY2020Q4	sigi	Line Of Credit Debt To Capitalization Ratio Requirement LineOfCreditDebtToCapitalizationRatioRequirement	0.35
CY2020Q4	us-gaap	Deferred Finance Costs Gross DeferredFinanceCostsGross	3300000	USD
CY2020Q4	sigi	Debt To Capitalization Ratio DebtToCapitalizationRatio	0.179
CY2020Q4	sigi	Federal Home Loan Bank Advances General Debt Obligations Disclosures Multiple Amount FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresMultipleAmount	20
CY2020Q4	sigi	Federal Home Loan Bank Advances General Debt Obligations Disclosures Multiple Amount FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresMultipleAmount	20
CY2018	us-gaap	Revenues Revenues	2586080000	USD
CY2020	us-gaap	Number Of Operating Segments NumberOfOperatingSegments	4
CY2020	sigi	Geographic Concentration Of Net Premiums Written Percentage GeographicConcentrationOfNetPremiumsWrittenPercentage	0.18
CY2019Q4	us-gaap	Goodwill Goodwill	7800000	USD
CY2020Q4	us-gaap	Goodwill Goodwill	7800000	USD
CY2020	us-gaap	Net Investment Income NetInvestmentIncome	227107000	USD
CY2019	us-gaap	Net Investment Income NetInvestmentIncome	222543000	USD
CY2018	us-gaap	Net Investment Income NetInvestmentIncome	195336000	USD
CY2020	us-gaap	Gain Loss On Investments GainLossOnInvestments	-4217000	USD
CY2019	us-gaap	Gain Loss On Investments GainLossOnInvestments	14422000	USD
CY2018	us-gaap	Gain Loss On Investments GainLossOnInvestments	-54923000	USD
CY2020	sigi	Total Investment Revenues TotalInvestmentRevenues	222890000	USD
CY2019	sigi	Total Investment Revenues TotalInvestmentRevenues	236965000	USD
CY2018	sigi	Total Investment Revenues TotalInvestmentRevenues	140413000	USD
CY2020	us-gaap	Revenues Revenues	2922274000	USD
CY2019	us-gaap	Revenues Revenues	2846491000	USD
CY2020	us-gaap	Net Investment Income NetInvestmentIncome	227107000	USD
CY2019	us-gaap	Net Investment Income NetInvestmentIncome	222543000	USD
CY2018	us-gaap	Net Investment Income NetInvestmentIncome	195336000	USD
CY2020	us-gaap	Gain Loss On Investments GainLossOnInvestments	-4217000	USD
CY2019	us-gaap	Gain Loss On Investments GainLossOnInvestments	14422000	USD
CY2018	us-gaap	Gain Loss On Investments GainLossOnInvestments	-54923000	USD
CY2020	sigi	Total Investment Revenues TotalInvestmentRevenues	222890000	USD
CY2019	sigi	Total Investment Revenues TotalInvestmentRevenues	236965000	USD
CY2018	sigi	Total Investment Revenues TotalInvestmentRevenues	140413000	USD
CY2020	sigi	Taxoninvestmentincome Taxoninvestmentincome	41609000	USD
CY2019	sigi	Taxoninvestmentincome Taxoninvestmentincome	45301000	USD
CY2018	sigi	Taxoninvestmentincome Taxoninvestmentincome	19560000	USD
CY2020	sigi	Total Investment Income After Federal Income Tax TotalInvestmentIncomeAfterFederalIncomeTax	181281000	USD
CY2019	sigi	Total Investment Income After Federal Income Tax TotalInvestmentIncomeAfterFederalIncomeTax	191664000	USD
CY2018	sigi	Total Investment Income After Federal Income Tax TotalInvestmentIncomeAfterFederalIncomeTax	120853000	USD
CY2020	sigi	Total Investment Revenues TotalInvestmentRevenues	222890000	USD
CY2019	sigi	Total Investment Revenues TotalInvestmentRevenues	236965000	USD
CY2018	sigi	Total Investment Revenues TotalInvestmentRevenues	140413000	USD
CY2020	us-gaap	Interest Expense Debt InterestExpenseDebt	30839000	USD
CY2019	us-gaap	Interest Expense Debt InterestExpenseDebt	33668000	USD
CY2018	us-gaap	Interest Expense Debt InterestExpenseDebt	24419000	USD
CY2020	us-gaap	Operating Costs And Expenses OperatingCostsAndExpenses	25412000	USD
CY2019	us-gaap	Operating Costs And Expenses OperatingCostsAndExpenses	30900000	USD
CY2018	us-gaap	Operating Costs And Expenses OperatingCostsAndExpenses	25446000	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	302988000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	336390000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	211721000	USD
CY2020	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	246355000	USD
CY2020	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	59862000	shares
CY2020	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	4.12
CY2020	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	431000	shares
CY2020	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	246355000	USD
CY2020	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	60293000	shares
CY2020	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	4.09
CY2019	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	271623000	USD
CY2019	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	59421000	shares
CY2019	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	4.57
CY2019	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	583000	shares
CY2019	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	271623000	USD
CY2019	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	60004000	shares
CY2019	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	4.53
CY2018	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	178939000	USD
CY2018	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	58950000	shares
CY2018	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	3.04
CY2018	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	763000	shares
CY2018	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	178939000	USD
CY2018	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	59713000	shares
CY2018	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	3.00
CY2020	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	63627000	USD
CY2019	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	70642000	USD
CY2018	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	44461000	USD
CY2020	sigi	Income Tax Reconciliation Tax Advantaged Interest Income IncomeTaxReconciliationTaxAdvantagedInterestIncome	4730000	USD
CY2019	sigi	Income Tax Reconciliation Tax Advantaged Interest Income IncomeTaxReconciliationTaxAdvantagedInterestIncome	4909000	USD
CY2018	sigi	Income Tax Reconciliation Tax Advantaged Interest Income IncomeTaxReconciliationTaxAdvantagedInterestIncome	5518000	USD
CY2020	us-gaap	Income Tax Reconciliation Deductions Dividends IncomeTaxReconciliationDeductionsDividends	514000	USD
CY2019	us-gaap	Income Tax Reconciliation Deductions Dividends IncomeTaxReconciliationDeductionsDividends	443000	USD
CY2018	us-gaap	Income Tax Reconciliation Deductions Dividends IncomeTaxReconciliationDeductionsDividends	647000	USD
CY2020	sigi	Federal Income Tax Expense Benefit Relatedto Executive Compensation FederalIncomeTaxExpenseBenefitRelatedtoExecutiveCompensation	-2246000	USD
CY2019	sigi	Federal Income Tax Expense Benefit Relatedto Executive Compensation FederalIncomeTaxExpenseBenefitRelatedtoExecutiveCompensation	-2985000	USD
CY2018	sigi	Federal Income Tax Expense Benefit Relatedto Executive Compensation FederalIncomeTaxExpenseBenefitRelatedtoExecutiveCompensation	-2279000	USD
CY2020	us-gaap	Income Tax Reconciliation Nondeductible Expense Share Based Compensation Cost IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost	-1846000	USD
CY2019	us-gaap	Income Tax Reconciliation Nondeductible Expense Share Based Compensation Cost IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost	-3253000	USD
CY2018	us-gaap	Income Tax Reconciliation Nondeductible Expense Share Based Compensation Cost IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost	-3093000	USD
CY2020	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	-2150000	USD
CY2019	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	-255000	USD
CY2018	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	-4700000	USD
CY2020	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	56633000	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	64767000	USD
CY2018	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	32782000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Loss Reserves DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves	54240000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Loss Reserves DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves	48193000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Unearned Premiums Reserve DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve	60842000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Unearned Premiums Reserve DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve	57004000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Employee Benefits DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits	8943000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Employee Benefits DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits	10646000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	5472000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	5727000	USD
CY2020Q4	sigi	Deferred Tax Assets Unrealized Losses On Securities Gross DeferredTaxAssetsUnrealizedLossesOnSecuritiesGross	6037000	USD
CY2019Q4	sigi	Deferred Tax Assets Unrealized Losses On Securities Gross DeferredTaxAssetsUnrealizedLossesOnSecuritiesGross	1059000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	7195000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	6478000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	142729000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	129107000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities Deferred Expense Deferred Policy Acquisition Cost DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost	60601000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Deferred Expense Deferred Policy Acquisition Cost DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost	56949000	USD
CY2020Q4	sigi	Deferred Tax Liabilities Investment Securities Unrealized Gain DeferredTaxLiabilitiesInvestmentSecuritiesUnrealizedGain	81142000	USD
CY2019Q4	sigi	Deferred Tax Liabilities Investment Securities Unrealized Gain DeferredTaxLiabilitiesInvestmentSecuritiesUnrealizedGain	45294000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities Other DeferredTaxLiabilitiesOther	14760000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Other DeferredTaxLiabilitiesOther	7576000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities Property Plant And Equipment DeferredTaxLiabilitiesPropertyPlantAndEquipment	13322000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Property Plant And Equipment DeferredTaxLiabilitiesPropertyPlantAndEquipment	12512000	USD
CY2020Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	169825000	USD
CY2019Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	122331000	USD
CY2020	sigi	Changein Net Deferred Federal Income Tax Assets ChangeinNetDeferredFederalIncomeTaxAssets	-33900000	USD
CY2020Q4	us-gaap	Defined Benefit Plan Weighted Average Asset Allocations DefinedBenefitPlanWeightedAverageAssetAllocations	1
CY2020Q4	sigi	Percent Hedge Against Projected Benefit Obligation PercentHedgeAgainstProjectedBenefitObligation	0.65
CY2019Q4	us-gaap	Defined Benefit Plan Weighted Average Asset Allocations DefinedBenefitPlanWeightedAverageAssetAllocations	1
CY2020	us-gaap	Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Expected Long Term Return On Assets DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets	0.0540
CY2020	sigi	Changein Defined Benefit Plan Discount Rate ChangeinDefinedBenefitPlanDiscountRate	65	USD
CY2020	us-gaap	Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Discount Rate DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate	0.0206
CY2020Q4	us-gaap	Defined Benefit Plan Weighted Average Asset Allocations DefinedBenefitPlanWeightedAverageAssetAllocations	1
CY2020	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	26823	shares
CY2020	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	16.71
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	26823	shares
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	16.71
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Gross ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross	0	shares
CY2020	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice	0
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures And Expirations In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod	0	shares
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures And Expirations In Period Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice	0
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	0	shares
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	0
CY2020	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P0Y
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue	0	USD
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	0	shares
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice	0
CY2020	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Weighted Average Remaining Contractual Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1	P0Y
CY2020Q4	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Intrinsic Value1 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1	0	USD
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	1300000	USD
CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	5200000	USD
CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	4500000	USD
CY2020	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	19800000	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	24500000	USD
CY2018	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	19300000	USD
CY2020	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	-5700000	USD
CY2019	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	-8200000	USD
CY2018	us-gaap	Employee Service Share Based Compensation Tax Benefit From Compensation Expense EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	-7000000.0	USD
CY2020	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	14100000	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	16300000	USD
CY2018	us-gaap	Allocated Share Based Compensation Expense Net Of Tax AllocatedShareBasedCompensationExpenseNetOfTax	12300000	USD
CY2020Q4	sigi	Preferred Stock Shares Issued Sigi PreferredStockSharesIssuedSIGI	8000000.0	shares
CY2020Q4	us-gaap	Preferred Stock Liquidation Preference Value PreferredStockLiquidationPreferenceValue	25000	USD
CY2018	sigi	Related Party Transactions Contributions To Foundation RelatedPartyTransactionsContributionsToFoundation	500000	USD
CY2020	sigi	Related Party Transactions Contributions To Foundation RelatedPartyTransactionsContributionsToFoundation	500000	USD
CY2019	sigi	Related Party Transactions Contributions To Foundation RelatedPartyTransactionsContributionsToFoundation	1300000	USD
CY2020	us-gaap	Costs And Expenses Related Party CostsAndExpensesRelatedParty	2000000.0	USD
CY2019	us-gaap	Costs And Expenses Related Party CostsAndExpensesRelatedParty	2200000	USD
CY2018	us-gaap	Costs And Expenses Related Party CostsAndExpensesRelatedParty	2000000.0	USD
CY2020Q4	us-gaap	Due To Related Parties Current And Noncurrent DueToRelatedPartiesCurrentAndNoncurrent	1300000	USD
CY2019Q4	us-gaap	Due To Related Parties Current And Noncurrent DueToRelatedPartiesCurrentAndNoncurrent	1100000	USD
CY2020	sigi	Finance Lease Expense FinanceLeaseExpense	565000	USD
CY2019	sigi	Finance Lease Expense FinanceLeaseExpense	1000000	USD
CY2020	us-gaap	Operating Lease Expense OperatingLeaseExpense	9498000	USD
CY2019	us-gaap	Operating Lease Expense OperatingLeaseExpense	8808000	USD
CY2020	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	550000	USD
CY2019	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	984000	USD
CY2020	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	15000	USD
CY2019	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	16000	USD
CY2020	us-gaap	Variable Lease Cost VariableLeaseCost	758000	USD
CY2019	us-gaap	Variable Lease Cost VariableLeaseCost	48000	USD
CY2020	us-gaap	Short Term Lease Cost ShortTermLeaseCost	2011000	USD
CY2019	us-gaap	Short Term Lease Cost ShortTermLeaseCost	2165000	USD
CY2020Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.023
CY2020Q4	us-gaap	Finance Lease Weighted Average Remaining Lease Term1 FinanceLeaseWeightedAverageRemainingLeaseTerm1	P2Y
CY2019Q4	us-gaap	Finance Lease Weighted Average Remaining Lease Term1 FinanceLeaseWeightedAverageRemainingLeaseTerm1	P2Y
CY2019Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.034
CY2020Q4	us-gaap	Finance Lease Weighted Average Discount Rate Percent FinanceLeaseWeightedAverageDiscountRatePercent	0.016
CY2019Q4	us-gaap	Finance Lease Weighted Average Discount Rate Percent FinanceLeaseWeightedAverageDiscountRatePercent	0.021
CY2020Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P8Y
CY2019Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P6Y
CY2020Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	40215000	USD
CY2019Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	26535000	USD
CY2020Q4	us-gaap	Finance Lease Right Of Use Asset FinanceLeaseRightOfUseAsset	502000	USD
CY2019Q4	us-gaap	Finance Lease Right Of Use Asset FinanceLeaseRightOfUseAsset	731000	USD
CY2020Q4	us-gaap	Finance Lease Liability FinanceLeaseLiability	508000	USD
CY2019Q4	us-gaap	Finance Lease Liability FinanceLeaseLiability	737000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Next Twelve Months FinanceLeaseLiabilityPaymentsDueNextTwelveMonths	330000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Next Twelve Months LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths	8372000	USD
CY2020Q4	sigi	Total Leases Due Next12 Months TotalLeasesDueNext12Months	8702000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Two FinanceLeaseLiabilityPaymentsDueYearTwo	127000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Two LesseeOperatingLeaseLiabilityPaymentsDueYearTwo	6788000	USD
CY2020Q4	sigi	Total Leases Due In Two Years TotalLeasesDueInTwoYears	6915000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Three FinanceLeaseLiabilityPaymentsDueYearThree	56000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Three LesseeOperatingLeaseLiabilityPaymentsDueYearThree	5411000	USD
CY2020Q4	sigi	Total Leases Due In Three Years TotalLeasesDueInThreeYears	5467000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Four FinanceLeaseLiabilityPaymentsDueYearFour	0	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Four LesseeOperatingLeaseLiabilityPaymentsDueYearFour	4690000	USD
CY2020Q4	sigi	Total Leases Due In Four Years TotalLeasesDueInFourYears	4690000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Undiscounted Excess Amount LesseeOperatingLeaseLiabilityUndiscountedExcessAmount	3393000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Five FinanceLeaseLiabilityPaymentsDueYearFive	0	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Five LesseeOperatingLeaseLiabilityPaymentsDueYearFive	3572000	USD
CY2020Q4	sigi	Total Leases Due In Five Years TotalLeasesDueInFiveYears	3572000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due After Year Five FinanceLeaseLiabilityPaymentsDueAfterYearFive	0	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due After Year Five LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive	16234000	USD
CY2020Q4	sigi	Total Leases Due Thereafter TotalLeasesDueThereafter	16234000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due FinanceLeaseLiabilityPaymentsDue	513000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	45067000	USD
CY2020Q4	sigi	Total Minimum Payment Required TotalMinimumPaymentRequired	45580000	USD
CY2020Q4	us-gaap	Finance Lease Liability Undiscounted Excess Amount FinanceLeaseLiabilityUndiscountedExcessAmount	5000	USD
CY2020Q4	sigi	Totalimputedinterest Totalimputedinterest	3398000	USD
CY2020Q4	sigi	Finance Lease Liability Not Yet Commenced FinanceLeaseLiabilityNotYetCommenced	0	USD
CY2020Q4	sigi	Operating Lease Liability Not Yet Commenced OperatingLeaseLiabilityNotYetCommenced	0	USD
CY2020Q4	sigi	Totalleaseliabilitynotyetcommenced Totalleaseliabilitynotyetcommenced	0	USD
CY2020Q4	us-gaap	Finance Lease Liability FinanceLeaseLiability	508000	USD
CY2020Q4	sigi	Totalleaseliability Totalleaseliability	42182000	USD
CY2019Q4	us-gaap	Finance Lease Liability Payments Due Next Twelve Months FinanceLeaseLiabilityPaymentsDueNextTwelveMonths	451000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Next Twelve Months LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths	8244000	USD
CY2019Q4	sigi	Total Leases Due Next12 Months TotalLeasesDueNext12Months	8695000	USD
CY2019Q4	us-gaap	Finance Lease Liability Payments Due Year Two FinanceLeaseLiabilityPaymentsDueYearTwo	248000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Two LesseeOperatingLeaseLiabilityPaymentsDueYearTwo	6168000	USD
CY2019Q4	sigi	Total Leases Due In Two Years TotalLeasesDueInTwoYears	6416000	USD
CY2019Q4	us-gaap	Finance Lease Liability Payments Due Year Three FinanceLeaseLiabilityPaymentsDueYearThree	54000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Three LesseeOperatingLeaseLiabilityPaymentsDueYearThree	4590000	USD
CY2019Q4	sigi	Total Leases Due In Three Years TotalLeasesDueInThreeYears	4644000	USD
CY2019Q4	us-gaap	Finance Lease Liability Payments Due Year Four FinanceLeaseLiabilityPaymentsDueYearFour	0	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Four LesseeOperatingLeaseLiabilityPaymentsDueYearFour	3329000	USD
CY2019Q4	sigi	Total Leases Due In Four Years TotalLeasesDueInFourYears	3329000	USD
CY2019Q4	us-gaap	Finance Lease Liability Payments Due Year Five FinanceLeaseLiabilityPaymentsDueYearFive	0	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Five LesseeOperatingLeaseLiabilityPaymentsDueYearFive	2920000	USD
CY2019Q4	sigi	Total Leases Due In Five Years TotalLeasesDueInFiveYears	2920000	USD
CY2019Q4	us-gaap	Finance Lease Liability Payments Due FinanceLeaseLiabilityPaymentsDue	753000	USD
CY2019Q4	us-gaap	Finance Lease Liability Payments Due After Year Five FinanceLeaseLiabilityPaymentsDueAfterYearFive	0	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due After Year Five LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive	8638000	USD
CY2019Q4	sigi	Total Leases Due Thereafter TotalLeasesDueThereafter	8638000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	33889000	USD
CY2019Q4	sigi	Total Minimum Payment Required TotalMinimumPaymentRequired	34642000	USD
CY2019Q4	us-gaap	Finance Lease Liability Undiscounted Excess Amount FinanceLeaseLiabilityUndiscountedExcessAmount	16000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Undiscounted Excess Amount LesseeOperatingLeaseLiabilityUndiscountedExcessAmount	2995000	USD
CY2019Q4	sigi	Totalimputedinterest Totalimputedinterest	3011000	USD
CY2019Q4	sigi	Finance Lease Liability Not Yet Commenced FinanceLeaseLiabilityNotYetCommenced	0	USD
CY2019Q4	sigi	Operating Lease Liability Not Yet Commenced OperatingLeaseLiabilityNotYetCommenced	3388000	USD
CY2019Q4	sigi	Totalleaseliabilitynotyetcommenced Totalleaseliabilitynotyetcommenced	3388000	USD
CY2019Q4	us-gaap	Finance Lease Liability FinanceLeaseLiability	737000	USD
CY2019Q4	sigi	Totalleaseliability Totalleaseliability	28243000	USD
CY2020Q4	sigi	Purchase Amount Of Annuities PurchaseAmountOfAnnuities	29100000	USD
CY2020Q4	sigi	Dividend Restriction New York DividendRestrictionNewYork	1
CY2020Q4	sigi	Statutory Accounting Practices Statutory Unassigned Surplus StatutoryAccountingPracticesStatutoryUnassignedSurplus	1559100000	USD
CY2019Q4	sigi	Statutory Accounting Practices Statutory Unassigned Surplus StatutoryAccountingPracticesStatutoryUnassignedSurplus	1396800000	USD
CY2020Q4	us-gaap	Statutory Accounting Practices Statutory Capital And Surplus Balance StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance	2124700000	USD
CY2019Q4	us-gaap	Statutory Accounting Practices Statutory Capital And Surplus Balance StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance	1932400000	USD
CY2020	us-gaap	Statutory Accounting Practices Statutory Net Income Amount StatutoryAccountingPracticesStatutoryNetIncomeAmount	242400000	USD
CY2019	us-gaap	Statutory Accounting Practices Statutory Net Income Amount StatutoryAccountingPracticesStatutoryNetIncomeAmount	292700000	USD
CY2018	us-gaap	Statutory Accounting Practices Statutory Net Income Amount StatutoryAccountingPracticesStatutoryNetIncomeAmount	210600000	USD
CY2020Q4	us-gaap	Statutory Accounting Practices Statutory Amount Available For Dividend Payments With Regulatory Approval StatutoryAccountingPracticesStatutoryAmountAvailableForDividendPaymentsWithRegulatoryApproval	241000000.0	USD
CY2020	sigi	Maximum Borrowing Percentage MaximumBorrowingPercentage	0.10
CY2020Q4	sigi	New Jersey Dividend Restriction NewJerseyDividendRestriction	0.10
CY2020Q4	sigi	New York Dividend Restriction NewYorkDividendRestriction	0.10
CY2019Q1	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	632573000	USD
CY2020Q2	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	630671000	USD
CY2020Q4	us-gaap	Statutory Accounting Practices Statutory Amount Available For Dividend Payments With Regulatory Approval StatutoryAccountingPracticesStatutoryAmountAvailableForDividendPaymentsWithRegulatoryApproval	241000000.0	USD
CY2020	us-gaap	Cash Dividends Paid To Parent Company CashDividendsPaidToParentCompany	105000000.0	USD
CY2020Q1	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	651703000	USD
CY2019Q2	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	642619000	USD
CY2020Q3	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	694541000	USD
CY2019Q3	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	653620000	USD
CY2020Q4	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	704899000	USD
CY2019Q4	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	668359000	USD
CY2020Q1	us-gaap	Net Investment Income NetInvestmentIncome	55967000	USD
CY2019Q1	us-gaap	Net Investment Income NetInvestmentIncome	50618000	USD
CY2020Q2	us-gaap	Net Investment Income NetInvestmentIncome	34444000	USD
CY2019Q2	us-gaap	Net Investment Income NetInvestmentIncome	58505000	USD
CY2020Q3	us-gaap	Net Investment Income NetInvestmentIncome	68185000	USD
CY2019Q3	us-gaap	Net Investment Income NetInvestmentIncome	55826000	USD
CY2020Q4	us-gaap	Net Investment Income NetInvestmentIncome	68511000	USD
CY2019Q4	us-gaap	Net Investment Income NetInvestmentIncome	57594000	USD
CY2020Q1	us-gaap	Gain Loss On Investments GainLossOnInvestments	-44666000	USD
CY2019Q1	us-gaap	Gain Loss On Investments GainLossOnInvestments	13451000	USD
CY2020Q2	us-gaap	Gain Loss On Investments GainLossOnInvestments	12649000	USD
CY2019Q2	us-gaap	Gain Loss On Investments GainLossOnInvestments	4027000	USD
CY2020Q3	us-gaap	Gain Loss On Investments GainLossOnInvestments	7721000	USD
CY2019Q3	us-gaap	Gain Loss On Investments GainLossOnInvestments	-2183000	USD
CY2020Q4	us-gaap	Gain Loss On Investments GainLossOnInvestments	20079000	USD
CY2019Q4	us-gaap	Gain Loss On Investments GainLossOnInvestments	-873000	USD
CY2020Q1	us-gaap	Other Income OtherIncome	1825000	USD
CY2019Q4	us-gaap	Other Income OtherIncome	3820000	USD
CY2019Q1	us-gaap	Other Income OtherIncome	2320000	USD
CY2020Q2	us-gaap	Other Income OtherIncome	4683000	USD
CY2019Q2	us-gaap	Other Income OtherIncome	3053000	USD
CY2020Q3	us-gaap	Other Income OtherIncome	6119000	USD
CY2019Q3	us-gaap	Other Income OtherIncome	3162000	USD
CY2020Q4	us-gaap	Other Income OtherIncome	4943000	USD
CY2020Q1	us-gaap	Revenues Revenues	664829000	USD
CY2019Q1	us-gaap	Revenues Revenues	698962000	USD
CY2020Q2	us-gaap	Revenues Revenues	682447000	USD
CY2019Q2	us-gaap	Revenues Revenues	708204000	USD
CY2020Q3	us-gaap	Revenues Revenues	776566000	USD
CY2019Q3	us-gaap	Revenues Revenues	710425000	USD
CY2020Q4	us-gaap	Revenues Revenues	798432000	USD
CY2019Q4	us-gaap	Revenues Revenues	728900000	USD
CY2020Q1	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	15997000	USD
CY2019Q1	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	73694000	USD
CY2020Q2	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	42693000	USD
CY2019Q2	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	72266000	USD
CY2020Q3	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	69875000	USD
CY2019Q2	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	90225000	USD
CY2020Q3	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	85257000	USD
CY2019Q3	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	71178000	USD
CY2020Q4	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	159041000	USD
CY2019Q4	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	101293000	USD
CY2020Q1	us-gaap	Net Income Loss NetIncomeLoss	15236000	USD
CY2019Q1	us-gaap	Net Income Loss NetIncomeLoss	61348000	USD
CY2020Q2	us-gaap	Net Income Loss NetIncomeLoss	34183000	USD
CY2019Q2	us-gaap	Net Income Loss NetIncomeLoss	72266000	USD
CY2020Q3	us-gaap	Net Income Loss NetIncomeLoss	69875000	USD
CY2019Q3	us-gaap	Net Income Loss NetIncomeLoss	56150000	USD
CY2020Q4	us-gaap	Net Income Loss NetIncomeLoss	127061000	USD
CY2019Q4	us-gaap	Net Income Loss NetIncomeLoss	81859000	USD
CY2020Q1	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	15236000	USD
CY2019Q1	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	61348000	USD
CY2020Q2	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	34183000	USD
CY2019Q3	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	56150000	USD
CY2020Q4	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	127061000	USD
CY2019Q4	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	81859000	USD
CY2020Q1	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.26
CY2019Q1	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.04
CY2020Q2	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.57
CY2019Q2	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.22
CY2020Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.17
CY2019Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.94
CY2020Q4	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.12
CY2019Q4	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.38
CY2020Q1	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.25
CY2019Q1	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.02
CY2020Q2	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.57
CY2019Q2	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.21
CY2020Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.16
CY2019Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.93
CY2020Q4	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.10
CY2019Q4	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.36
CY2020Q4	us-gaap	Summary Of Investments Other Than Investments In Related Parties Cost SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost	7114400000	USD
CY2020Q4	us-gaap	Summary Of Investments Other Than Investments In Related Parties Carrying Amount SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount	7505599000	USD
CY2020Q4	us-gaap	Supplementary Insurance Information Deferred Policy Acquisition Costs SupplementaryInsuranceInformationDeferredPolicyAcquisitionCosts	288578000	USD
CY2020Q4	us-gaap	Supplementary Insurance Information Liability For Future Policy Benefits Losses Claims And Loss Expense Reserves SupplementaryInsuranceInformationLiabilityForFuturePolicyBenefitsLossesClaimsAndLossExpenseReserves	4260355000	USD
CY2020Q4	us-gaap	Supplementary Insurance Information Unearned Premiums SupplementaryInsuranceInformationUnearnedPremiums	1618271000	USD
CY2020	us-gaap	Supplementary Insurance Information Premium Revenue SupplementaryInsuranceInformationPremiumRevenue	2681814000	USD
CY2020	us-gaap	Supplementary Insurance Information Net Investment Income SupplementaryInsuranceInformationNetInvestmentIncome	222890000	USD
CY2020	us-gaap	Supplementary Insurance Information Benefits Claims Losses And Settlement Expense SupplementaryInsuranceInformationBenefitsClaimsLossesAndSettlementExpense	1635823000	USD
CY2020	us-gaap	Supplementary Insurance Information Amortization Of Deferred Policy Acquisition Costs SupplementaryInsuranceInformationAmortizationOfDeferredPolicyAcquisitionCosts	560271000	USD
CY2020	us-gaap	Supplementary Insurance Information Other Operating Expense SupplementaryInsuranceInformationOtherOperatingExpense	349371000	USD
CY2020	us-gaap	Supplementary Insurance Information Premiums Written SupplementaryInsuranceInformationPremiumsWritten	2773092000	USD
CY2020	us-gaap	Supplementary Insurance Information Other Operating Expense SupplementaryInsuranceInformationOtherOperatingExpense	349371000	USD
CY2020	us-gaap	Other Underwriting Expense OtherUnderwritingExpense	366941000	USD
CY2020	us-gaap	Supplementary Insurance Information Other Operating Expense SupplementaryInsuranceInformationOtherOperatingExpense	349371000	USD
CY2019Q4	us-gaap	Supplementary Insurance Information Deferred Policy Acquisition Costs SupplementaryInsuranceInformationDeferredPolicyAcquisitionCosts	271186000	USD
CY2019Q4	us-gaap	Supplementary Insurance Information Liability For Future Policy Benefits Losses Claims And Loss Expense Reserves SupplementaryInsuranceInformationLiabilityForFuturePolicyBenefitsLossesClaimsAndLossExpenseReserves	4067163000	USD
CY2019Q4	us-gaap	Supplementary Insurance Information Unearned Premiums SupplementaryInsuranceInformationUnearnedPremiums	1523167000	USD
CY2019	us-gaap	Supplementary Insurance Information Premium Revenue SupplementaryInsuranceInformationPremiumRevenue	2597171000	USD
CY2019	us-gaap	Supplementary Insurance Information Net Investment Income SupplementaryInsuranceInformationNetInvestmentIncome	236965000	USD
CY2019	us-gaap	Supplementary Insurance Information Benefits Claims Losses And Settlement Expense SupplementaryInsuranceInformationBenefitsClaimsLossesAndSettlementExpense	1551491000	USD
CY2019	us-gaap	Supplementary Insurance Information Amortization Of Deferred Policy Acquisition Costs SupplementaryInsuranceInformationAmortizationOfDeferredPolicyAcquisitionCosts	535973000	USD
CY2019	us-gaap	Supplementary Insurance Information Other Operating Expense SupplementaryInsuranceInformationOtherOperatingExpense	345714000	USD
CY2019	us-gaap	Supplementary Insurance Information Premiums Written SupplementaryInsuranceInformationPremiumsWritten	2679424000	USD
CY2019	us-gaap	Supplementary Insurance Information Other Operating Expense SupplementaryInsuranceInformationOtherOperatingExpense	345714000	USD
CY2019	us-gaap	Other Underwriting Expense OtherUnderwritingExpense	358069000	USD
CY2019	us-gaap	Supplementary Insurance Information Other Operating Expense SupplementaryInsuranceInformationOtherOperatingExpense	345714000	USD
CY2018Q4	us-gaap	Supplementary Insurance Information Deferred Policy Acquisition Costs SupplementaryInsuranceInformationDeferredPolicyAcquisitionCosts	252612000	USD
CY2018Q4	us-gaap	Supplementary Insurance Information Liability For Future Policy Benefits Losses Claims And Loss Expense Reserves SupplementaryInsuranceInformationLiabilityForFuturePolicyBenefitsLossesClaimsAndLossExpenseReserves	3893868000	USD
CY2018Q4	us-gaap	Supplementary Insurance Information Unearned Premiums SupplementaryInsuranceInformationUnearnedPremiums	1431932000	USD
CY2018	us-gaap	Supplementary Insurance Information Premium Revenue SupplementaryInsuranceInformationPremiumRevenue	2436229000	USD
CY2018	us-gaap	Supplementary Insurance Information Net Investment Income SupplementaryInsuranceInformationNetInvestmentIncome	140413000	USD
CY2018	us-gaap	Supplementary Insurance Information Benefits Claims Losses And Settlement Expense SupplementaryInsuranceInformationBenefitsClaimsLossesAndSettlementExpense	1498134000	USD
CY2018	us-gaap	Supplementary Insurance Information Amortization Of Deferred Policy Acquisition Costs SupplementaryInsuranceInformationAmortizationOfDeferredPolicyAcquisitionCosts	495042000	USD
CY2018	us-gaap	Supplementary Insurance Information Other Operating Expense SupplementaryInsuranceInformationOtherOperatingExpense	321880000	USD
CY2018	us-gaap	Supplementary Insurance Information Premiums Written SupplementaryInsuranceInformationPremiumsWritten	2514286000	USD
CY2018	us-gaap	Supplementary Insurance Information Other Operating Expense SupplementaryInsuranceInformationOtherOperatingExpense	321880000	USD
CY2018	us-gaap	Other Underwriting Expense OtherUnderwritingExpense	331318000	USD
CY2018	us-gaap	Supplementary Insurance Information Other Operating Expense SupplementaryInsuranceInformationOtherOperatingExpense	321880000	USD
CY2020	us-gaap	Direct Premiums Earned DirectPremiumsEarned	3108687000	USD
CY2020	us-gaap	Assumed Premiums Earned AssumedPremiumsEarned	25010000	USD
CY2020	us-gaap	Ceded Premiums Earned CededPremiumsEarned	451883000	USD
CY2020	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	2681814000	USD
CY2020	us-gaap	Premiums Percentage Assumed To Net PremiumsPercentageAssumedToNet	0.01
CY2019	us-gaap	Direct Premiums Earned DirectPremiumsEarned	2993157000	USD
CY2019	us-gaap	Assumed Premiums Earned AssumedPremiumsEarned	24399000	USD
CY2019	us-gaap	Ceded Premiums Earned CededPremiumsEarned	420385000	USD
CY2019	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	2597171000	USD
CY2019	us-gaap	Premiums Percentage Assumed To Net PremiumsPercentageAssumedToNet	0.01
CY2018	us-gaap	Direct Premiums Earned DirectPremiumsEarned	2808764000	USD
CY2018	us-gaap	Assumed Premiums Earned AssumedPremiumsEarned	25831000	USD
CY2018	us-gaap	Ceded Premiums Earned CededPremiumsEarned	398366000	USD
CY2018	us-gaap	Premiums Earned Net Property And Casualty PremiumsEarnedNetPropertyAndCasualty	2436229000	USD
CY2018	us-gaap	Premiums Percentage Assumed To Net PremiumsPercentageAssumedToNet	0.01
CY2019Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	10800000	USD
CY2018Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	13900000	USD
CY2017Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	14600000	USD
CY2020	us-gaap	Valuation Allowances And Reserves Charged To Cost And Expense ValuationAllowancesAndReservesChargedToCostAndExpense	17576000	USD
CY2019	us-gaap	Valuation Allowances And Reserves Charged To Cost And Expense ValuationAllowancesAndReservesChargedToCostAndExpense	2730000	USD
CY2018	us-gaap	Valuation Allowances And Reserves Charged To Cost And Expense ValuationAllowancesAndReservesChargedToCostAndExpense	4022000	USD
CY2020	us-gaap	Valuation Allowances And Reserves Deductions ValuationAllowancesAndReservesDeductions	-3754000	USD
CY2019	us-gaap	Valuation Allowances And Reserves Deductions ValuationAllowancesAndReservesDeductions	-5830000	USD
CY2018	us-gaap	Valuation Allowances And Reserves Deductions ValuationAllowancesAndReservesDeductions	-4722000	USD
CY2020Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	22777000	USD
CY2019Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	10800000	USD
CY2018Q4	us-gaap	Valuation Allowances And Reserves Balance ValuationAllowancesAndReservesBalance	13900000	USD

Files In Submission

Name	View Source	Status
0000230557-21-000009-index-headers.html	Edgar Link	pending
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sigi-ex991_12312020xq4.htm	Edgar Link	pending

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$SIGI
Selective Insurance Group Inc Stock $sigi

2021 Q1 Form 10-K Financial Statement

#000023055721000009 Filed on February 12, 2021

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$SIGI Selective Insurance Group Inc Stock $sigi

2021 Q1 Form 10-K Financial Statement

#000023055721000009 Filed on February 12, 2021

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$SIGI
Selective Insurance Group Inc Stock $sigi