$WTM
WHITE MOUNTAINS INSURANCE GROUP LTD Stock $wtm

$wtm Overview Financials Valuation SEC Filings Company Website

2019 Form 10-K Financial Statement

#000077686720000009 Filed on March 02, 2020

Income Statement

Concept	2019	2018 Q4	2018 Q3
Revenue	$893.4M	$6.000M	$198.7M
YoY Change	142.05%	-94.74%	127.09%
Cost Of Revenue	$48.10M		$62.90M
YoY Change	-80.66%		90.03%
Gross Profit	$845.3M		$135.8M
YoY Change	602.08%		149.63%
Gross Profit Margin	94.62%		68.34%
Selling, General & Admin	$438.7M	$139.2M	$142.3M
YoY Change	-14.53%	35.67%	90.24%
% of Gross Profit	51.9%		104.79%
Research & Development
YoY Change
% of Gross Profit
Depreciation & Amortization	$29.80M	$5.900M	$1.200M
YoY Change	15.95%	0.0%	33.33%
% of Gross Profit	3.53%		0.88%
Operating Expenses	$466.6M	$146.6M	$151.0M
YoY Change	-13.24%	35.36%	93.09%
Operating Profit	$378.7M		-$15.20M
YoY Change	-190.73%		-36.13%
Interest Expense	-$19.70M	-$3.500M	-$3.400M
YoY Change	107.37%	600.0%	277.78%
% of Operating Profit	-5.2%
Other Income/Expense, Net
YoY Change
Pretax Income	$405.1M	-$144.1M	$44.30M
YoY Change	-327.33%	-2871.15%	427.38%
Income Tax	$29.30M	-$3.600M	-$3.600M
% Of Pretax Income	7.23%		-8.13%
Net Earnings	$376.6M	-$137.5M	$30.60M
YoY Change	-296.76%	-711.11%	-94.45%
Net Earnings / Revenue	42.15%	-2291.67%	15.4%
Basic Earnings Per Share	$130.30	-$43.24	$12.83
Diluted Earnings Per Share	$130.30	-$43.24	$12.83
COMMON SHARES
Basic Shares Outstanding	3.141M shares	3.181M shares	3.139M shares
Diluted Shares Outstanding	3.141M shares		3.139M shares

Balance Sheet

Concept	2019	2018 Q4	2018 Q3
SHORT-TERM ASSETS
Cash & Short-Term Investments
YoY Change
Cash & Equivalents		$110.3M	$112.2M
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	$23.00M
YoY Change
Goodwill		$379.9M	$321.4M
YoY Change		1366.8%	1140.93%
Intangibles
YoY Change
Long-Term Investments	$2.947B	$2.543B	$2.679B
YoY Change	15.89%	-24.79%	-18.35%
Other Assets	$3.000M	$3.000M	$3.000M
YoY Change	0.0%	0.0%
Total Long-Term Assets
YoY Change
TOTAL ASSETS
Total Short-Term Assets
Total Long-Term Assets
Total Assets	$3.983B	$3.363B	$3.423B
YoY Change	18.44%	-8.11%	-5.89%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable
YoY Change
Accrued Expenses	$100.0M	$39.00M	$40.00M
YoY Change	156.41%	-51.85%	-48.05%
Deferred Revenue
YoY Change
Short-Term Debt	$0.00	$0.00	$0.00
YoY Change
Long-Term Debt Due
YoY Change
Total Short-Term Liabilities	$100.0M	$73.00M	$98.00M
YoY Change	36.99%	-35.4%	-22.83%
LONG-TERM LIABILITIES
Long-Term Debt	$284.0M	$193.0M	$164.0M
YoY Change	47.15%	704.17%	1722.22%
Other Long-Term Liabilities	$155.0M	$125.0M	$107.0M
YoY Change	24.0%	400.0%	137.78%
Total Long-Term Liabilities	$439.0M	$318.0M	$271.0M
YoY Change	38.05%	548.98%	401.85%
TOTAL LIABILITIES
Total Short-Term Liabilities	$100.0M	$73.00M	$98.00M
Total Long-Term Liabilities	$439.0M	$318.0M	$271.0M
Total Liabilities	$722.0M	$644.4M	$583.2M
YoY Change	38.85%	115.95%	94.46%
SHAREHOLDERS EQUITY
Retained Earnings		$2.265B	$2.407B
YoY Change		-19.78%	-14.05%
Common Stock		$584.0M	$579.5M
YoY Change		-12.91%	-13.42%
Preferred Stock
YoY Change
Treasury Stock (at cost)
YoY Change
Treasury Stock Shares
Shareholders Equity	$3.262B	$2.843B	$2.984B
YoY Change
Total Liabilities & Shareholders Equity	$3.983B	$3.363B	$3.423B
YoY Change	18.44%	-8.11%	-5.89%

Cashflow Statement

Concept	2019	2018 Q4	2018 Q3
OPERATING ACTIVITIES
Net Income	$376.6M	-$137.5M	$30.60M
YoY Change	-296.76%	-711.11%	-94.45%
Depreciation, Depletion And Amortization	$29.80M	$5.900M	$1.200M
YoY Change	15.95%	0.0%	33.33%
Cash From Operating Activities	-$120.5M	-$600.0K	-$5.100M
YoY Change	287.46%	-130.0%	-107.13%
INVESTING ACTIVITIES
Capital Expenditures
YoY Change
Acquisitions
YoY Change
Other Investing Activities	$27.30M	-$49.20M	-$20.50M
YoY Change	-216.67%	-336.54%	-102.79%
Cash From Investing Activities	-$59.90M	-$49.20M	-$18.10M
YoY Change	-113.04%	-336.54%	-102.47%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net	$0.00
YoY Change	-100.0%
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	$230.5M	47.90M	4.900M
YoY Change	-155.64%	86.38%	-100.69%
NET CHANGE
Cash From Operating Activities	-$120.5M	-600.0K	-5.100M
Cash From Investing Activities	-$59.90M	-49.20M	-18.10M
Cash From Financing Activities	$230.5M	47.90M	4.900M
Net Change In Cash	$50.70M	-1.900M	-18.30M
YoY Change	284.09%	-103.92%	-119.96%
FREE CASH FLOW
Cash From Operating Activities	-$120.5M	-$600.0K	-$5.100M
Capital Expenditures
Free Cash Flow
YoY Change

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2019	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2019Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	50000000	shares
CY2019	dei	Amendment Flag AmendmentFlag	false
CY2019	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
CY2019	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2019
CY2019	dei	Entity Central Index Key EntityCentralIndexKey	0000776867
CY2017	us-gaap	Cash Acquired From Acquisition CashAcquiredFromAcquisition	0	USD
CY2018	us-gaap	Cash Acquired From Acquisition CashAcquiredFromAcquisition	90900000	USD
CY2019	us-gaap	Cash Acquired From Acquisition CashAcquiredFromAcquisition	12800000	USD
CY2018Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	1.00
CY2019Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	1.00
CY2018Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	50000000	shares
CY2018Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	3173115	shares
CY2019Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	3185353	shares
CY2018Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	3173115	shares
CY2019Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	3185353	shares
CY2016Q4	us-gaap	Restricted Cash And Cash Equivalents At Carrying Value RestrictedCashAndCashEquivalentsAtCarryingValue	0	USD
CY2017Q4	us-gaap	Restricted Cash And Cash Equivalents At Carrying Value RestrictedCashAndCashEquivalentsAtCarryingValue	0	USD
CY2018Q4	us-gaap	Restricted Cash And Cash Equivalents At Carrying Value RestrictedCashAndCashEquivalentsAtCarryingValue	50000000	USD
CY2019Q4	us-gaap	Restricted Cash And Cash Equivalents At Carrying Value RestrictedCashAndCashEquivalentsAtCarryingValue	56300000	USD
CY2017	wtm	Restricted Cash Acquired RestrictedCashAcquired	0	USD
CY2018	wtm	Restricted Cash Acquired RestrictedCashAcquired	53400000	USD
CY2019	wtm	Restricted Cash Acquired RestrictedCashAcquired	0	USD
CY2019	dei	Document Type DocumentType	10-K
CY2019	dei	Document Annual Report DocumentAnnualReport	true
CY2019	dei	Document Period End Date DocumentPeriodEndDate	2019-12-31
CY2019	dei	Document Transition Report DocumentTransitionReport	false
CY2019	dei	Entity File Number EntityFileNumber	1-8993
CY2019	dei	Entity Registrant Name EntityRegistrantName	WHITE MOUNTAINS INSURANCE GROUP, LTD.
CY2019	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	D0
CY2019	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	94-2708455
CY2019	dei	Entity Address Address Line1 EntityAddressAddressLine1	80 South Main Street
CY2019	dei	Entity Address City Or Town EntityAddressCityOrTown	Hanover,
CY2019	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	03755-2053
CY2019	dei	Entity Address State Or Province EntityAddressStateOrProvince	NH
CY2019	dei	City Area Code CityAreaCode	603
CY2019	dei	Local Phone Number LocalPhoneNumber	640-2200
CY2019	dei	Security12b Title Security12bTitle	Common Shares, par value $1.00
CY2019	dei	Trading Symbol TradingSymbol	WTM
CY2019	dei	Security Exchange Name SecurityExchangeName	NYSE
CY2019	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	Yes
CY2019	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
CY2019	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2019	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
CY2019	dei	Entity Filer Category EntityFilerCategory	Large Accelerated Filer
CY2019	dei	Entity Small Business EntitySmallBusiness	false
CY2019	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
CY2019	dei	Entity Shell Company EntityShellCompany	false
CY2019Q2	dei	Entity Public Float EntityPublicFloat	3169884560	USD
CY2020Q1	dei	Entity Common Stock Shares Outstanding EntityCommonStockSharesOutstanding	3193509	shares
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CY2018Q4	us-gaap	Assets Assets	3362600000	USD
CY2019Q4	us-gaap	Liabilities Liabilities	838500000	USD
CY2018Q4	us-gaap	Liabilities Liabilities	644400000	USD
CY2019Q4	us-gaap	Common Stock Value CommonStockValue	3200000	USD
CY2018Q4	us-gaap	Common Stock Value CommonStockValue	3200000	USD
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CY2019Q4	us-gaap	Stockholders Equity StockholdersEquity	3261500000	USD
CY2018Q4	us-gaap	Stockholders Equity StockholdersEquity	2843100000	USD
CY2019Q4	us-gaap	Minority Interest MinorityInterest	-116800000	USD
CY2018Q4	us-gaap	Minority Interest MinorityInterest	-124900000	USD
CY2019Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	3144700000	USD
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CY2018	us-gaap	Revenues Revenues	369100000	USD
CY2017	us-gaap	Revenues Revenues	373800000	USD
CY2019	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	488300000	USD
CY2018	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	547300000	USD
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CY2019	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	414500000	USD
CY2018	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-141200000	USD
CY2017	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	627200000	USD
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CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-1400000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-4800000	USD
CY2017	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	300000	USD
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CY2019	us-gaap	Increase Decrease In Trading Securities IncreaseDecreaseInTradingSecurities	118400000	USD
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CY2019	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	24800000	USD
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CY2017	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-11400000	USD
CY2019	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	0	USD
CY2018	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	0	USD
CY2017	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	20500000	USD
CY2019	us-gaap	Discontinued Operation Gain Loss On Disposal Of Discontinued Operation Net Of Tax DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax	800000	USD
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CY2019	us-gaap	Increase Decrease In Unearned Premiums IncreaseDecreaseInUnearnedPremiums	35800000	USD
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CY2017	us-gaap	Increase Decrease In Trading Securities IncreaseDecreaseInTradingSecurities	0	USD
CY2019	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	-18700000	USD
CY2018	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	27100000	USD
CY2017	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	20800000	USD
CY2019	us-gaap	Net Cash Provided By Used In Operating Activities Continuing Operations NetCashProvidedByUsedInOperatingActivitiesContinuingOperations	-120500000	USD
CY2018	us-gaap	Net Cash Provided By Used In Operating Activities Continuing Operations NetCashProvidedByUsedInOperatingActivitiesContinuingOperations	-31100000	USD
CY2017	us-gaap	Net Cash Provided By Used In Operating Activities Continuing Operations NetCashProvidedByUsedInOperatingActivitiesContinuingOperations	-62400000	USD
CY2019	us-gaap	Cash Provided By Used In Operating Activities Discontinued Operations CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations	0	USD
CY2018	us-gaap	Cash Provided By Used In Operating Activities Discontinued Operations CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations	0	USD
CY2017	us-gaap	Cash Provided By Used In Operating Activities Discontinued Operations CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations	157000000.0	USD
CY2019	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-120500000	USD
CY2018	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-31100000	USD
CY2017	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	94600000	USD
CY2019	us-gaap	Payments For Proceeds From Short Term Investments PaymentsForProceedsFromShortTermInvestments	-21900000	USD
CY2018	us-gaap	Payments For Proceeds From Short Term Investments PaymentsForProceedsFromShortTermInvestments	39000000.0	USD
CY2017	us-gaap	Payments For Proceeds From Short Term Investments PaymentsForProceedsFromShortTermInvestments	1700000	USD
CY2019	wtm	Proceeds From Sale Of Trading Fixed Maturity And Convertible Fixed Maturity Investments ProceedsFromSaleOfTradingFixedMaturityAndConvertibleFixedMaturityInvestments	334300000	USD
CY2018	wtm	Proceeds From Sale Of Trading Fixed Maturity And Convertible Fixed Maturity Investments ProceedsFromSaleOfTradingFixedMaturityAndConvertibleFixedMaturityInvestments	1848500000	USD
CY2017	wtm	Proceeds From Sale Of Trading Fixed Maturity And Convertible Fixed Maturity Investments ProceedsFromSaleOfTradingFixedMaturityAndConvertibleFixedMaturityInvestments	2124400000	USD
CY2019	wtm	Proceeds From Maturities Of Trading Fixed Maturity And Convertible Fixed Maturity Investments ProceedsFromMaturitiesOfTradingFixedMaturityAndConvertibleFixedMaturityInvestments	119300000	USD
CY2018	wtm	Proceeds From Maturities Of Trading Fixed Maturity And Convertible Fixed Maturity Investments ProceedsFromMaturitiesOfTradingFixedMaturityAndConvertibleFixedMaturityInvestments	141000000.0	USD
CY2017	wtm	Proceeds From Maturities Of Trading Fixed Maturity And Convertible Fixed Maturity Investments ProceedsFromMaturitiesOfTradingFixedMaturityAndConvertibleFixedMaturityInvestments	213400000	USD
CY2019	wtm	Proceeds From Sale Of Trading Common Equity Securities ProceedsFromSaleOfTradingCommonEquitySecurities	467200000	USD
CY2018	wtm	Proceeds From Sale Of Trading Common Equity Securities ProceedsFromSaleOfTradingCommonEquitySecurities	169900000	USD
CY2017	wtm	Proceeds From Sale Of Trading Common Equity Securities ProceedsFromSaleOfTradingCommonEquitySecurities	424100000	USD
CY2019	us-gaap	Proceeds From Sale And Maturity Of Other Investments ProceedsFromSaleAndMaturityOfOtherInvestments	29400000	USD
CY2018	us-gaap	Proceeds From Sale And Maturity Of Other Investments ProceedsFromSaleAndMaturityOfOtherInvestments	5000000.0	USD
CY2017	us-gaap	Proceeds From Sale And Maturity Of Other Investments ProceedsFromSaleAndMaturityOfOtherInvestments	29400000	USD
CY2019	us-gaap	Proceeds From Divestiture Of Interest In Subsidiaries And Affiliates ProceedsFromDivestitureOfInterestInSubsidiariesAndAffiliates	0	USD
CY2018	us-gaap	Proceeds From Divestiture Of Interest In Subsidiaries And Affiliates ProceedsFromDivestitureOfInterestInSubsidiariesAndAffiliates	0	USD
CY2017	us-gaap	Proceeds From Divestiture Of Interest In Subsidiaries And Affiliates ProceedsFromDivestitureOfInterestInSubsidiariesAndAffiliates	1131000000.0	USD
CY2019	us-gaap	Payments To Acquire Other Investments PaymentsToAcquireOtherInvestments	177700000	USD
CY2018	us-gaap	Payments To Acquire Other Investments PaymentsToAcquireOtherInvestments	95900000	USD
CY2017	us-gaap	Payments To Acquire Other Investments PaymentsToAcquireOtherInvestments	84100000	USD
CY2019	wtm	Distributionfrom Contributionsto Discontinued Operationsfor Investing Activities DistributionfromContributionstoDiscontinuedOperationsforInvestingActivities	0	USD
CY2018	wtm	Distributionfrom Contributionsto Discontinued Operationsfor Investing Activities DistributionfromContributionstoDiscontinuedOperationsforInvestingActivities	0	USD
CY2017	wtm	Distributionfrom Contributionsto Discontinued Operationsfor Investing Activities DistributionfromContributionstoDiscontinuedOperationsforInvestingActivities	-167700000	USD
CY2019	wtm	Payments To Acquire Trading Common Equity Securities PaymentsToAcquireTradingCommonEquitySecurities	29800000	USD
CY2018	wtm	Payments To Acquire Trading Common Equity Securities PaymentsToAcquireTradingCommonEquitySecurities	328300000	USD
CY2017	wtm	Payments To Acquire Trading Common Equity Securities PaymentsToAcquireTradingCommonEquitySecurities	881200000	USD
CY2019	wtm	Payments To Acquire Trading Fixed Maturity And Convertible Fixed Maturity Investments PaymentsToAcquireTradingFixedMaturityAndConvertibleFixedMaturityInvestments	539200000	USD
CY2018	wtm	Payments To Acquire Trading Fixed Maturity And Convertible Fixed Maturity Investments PaymentsToAcquireTradingFixedMaturityAndConvertibleFixedMaturityInvestments	970200000	USD
CY2017	wtm	Payments To Acquire Trading Fixed Maturity And Convertible Fixed Maturity Investments PaymentsToAcquireTradingFixedMaturityAndConvertibleFixedMaturityInvestments	2365200000	USD
CY2019	us-gaap	Payments To Acquire Businesses And Interest In Affiliates PaymentsToAcquireBusinessesAndInterestInAffiliates	258000000.0	USD
CY2018	us-gaap	Payments To Acquire Businesses And Interest In Affiliates PaymentsToAcquireBusinessesAndInterestInAffiliates	295200000	USD
CY2017	us-gaap	Payments To Acquire Businesses And Interest In Affiliates PaymentsToAcquireBusinessesAndInterestInAffiliates	27600000	USD
CY2019	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	27300000	USD
CY2018	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-23400000	USD
CY2017	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	14700000	USD
CY2019	us-gaap	Net Cash Provided By Used In Investing Activities Continuing Operations NetCashProvidedByUsedInInvestingActivitiesContinuingOperations	-59900000	USD
CY2018	us-gaap	Net Cash Provided By Used In Investing Activities Continuing Operations NetCashProvidedByUsedInInvestingActivitiesContinuingOperations	459200000	USD
CY2017	us-gaap	Net Cash Provided By Used In Investing Activities Continuing Operations NetCashProvidedByUsedInInvestingActivitiesContinuingOperations	715500000	USD
CY2019	us-gaap	Cash Provided By Used In Investing Activities Discontinued Operations CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations	0	USD
CY2018	us-gaap	Cash Provided By Used In Investing Activities Discontinued Operations CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations	0	USD
CY2017	us-gaap	Cash Provided By Used In Investing Activities Discontinued Operations CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations	3000000.0	USD
CY2019	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-59900000	USD
CY2018	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	459200000	USD
CY2017	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	718500000	USD
CY2019	us-gaap	Proceeds From Lines Of Credit ProceedsFromLinesOfCredit	206400000	USD
CY2019	wtm	Acquisitionsof Additional Shares From Non Controlling Interest AcquisitionsofAdditionalSharesFromNonControllingInterest	0	USD
CY2018	wtm	Acquisitionsof Additional Shares From Non Controlling Interest AcquisitionsofAdditionalSharesFromNonControllingInterest	1700000	USD
CY2017	wtm	Acquisitionsof Additional Shares From Non Controlling Interest AcquisitionsofAdditionalSharesFromNonControllingInterest	700000	USD
CY2019	wtm	Distributionsfromcontributionstodiscontinuedoperations Distributionsfromcontributionstodiscontinuedoperations	0	USD
CY2018	wtm	Distributionsfromcontributionstodiscontinuedoperations Distributionsfromcontributionstodiscontinuedoperations	0	USD
CY2017	wtm	Distributionsfromcontributionstodiscontinuedoperations Distributionsfromcontributionstodiscontinuedoperations	45200000	USD
CY2018	us-gaap	Proceeds From Lines Of Credit ProceedsFromLinesOfCredit	84100000	USD
CY2017	us-gaap	Proceeds From Lines Of Credit ProceedsFromLinesOfCredit	376000000.0	USD
CY2019	us-gaap	Repayments Of Debt RepaymentsOfDebt	22900000	USD
CY2018	us-gaap	Repayments Of Debt RepaymentsOfDebt	15400000	USD
CY2017	us-gaap	Repayments Of Debt RepaymentsOfDebt	365000000.0	USD
CY2019	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	3200000	USD
CY2018	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	3800000	USD
CY2017	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	4600000	USD
CY2019	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	0	USD
CY2018	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	511900000	USD
CY2017	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	714600000	USD
CY2019	wtm	Repurchaseof Shares From Noncontrolling Interest Shareholders RepurchaseofSharesFromNoncontrollingInterestShareholders	21100000	USD
CY2018	wtm	Repurchaseof Shares From Noncontrolling Interest Shareholders RepurchaseofSharesFromNoncontrollingInterestShareholders	0	USD
CY2017	wtm	Repurchaseof Shares From Noncontrolling Interest Shareholders RepurchaseofSharesFromNoncontrollingInterestShareholders	0	USD
CY2019	wtm	Proceeds From Issuanceof Sharesto Noncontrolling Interest Shareholders ProceedsFromIssuanceofSharestoNoncontrollingInterestShareholders	62700000	USD
CY2018	wtm	Proceeds From Issuanceof Sharesto Noncontrolling Interest Shareholders ProceedsFromIssuanceofSharestoNoncontrollingInterestShareholders	0	USD
CY2017	wtm	Proceeds From Issuanceof Sharesto Noncontrolling Interest Shareholders ProceedsFromIssuanceofSharestoNoncontrollingInterestShareholders	0	USD
CY2019	us-gaap	Proceeds From Partnership Contribution ProceedsFromPartnershipContribution	1900000	USD
CY2018	us-gaap	Proceeds From Partnership Contribution ProceedsFromPartnershipContribution	1300000	USD
CY2017	us-gaap	Proceeds From Partnership Contribution ProceedsFromPartnershipContribution	500000	USD
CY2019	us-gaap	Payments Of Capital Distribution PaymentsOfCapitalDistribution	28100000	USD
CY2018	us-gaap	Payments Of Capital Distribution PaymentsOfCapitalDistribution	6000000.0	USD
CY2017	us-gaap	Payments Of Capital Distribution PaymentsOfCapitalDistribution	2000000.0	USD
CY2019	wtm	Paymentofcontingentconsideration Paymentofcontingentconsideration	7600000	USD
CY2018	wtm	Paymentofcontingentconsideration Paymentofcontingentconsideration	2600000	USD
CY2017	wtm	Paymentofcontingentconsideration Paymentofcontingentconsideration	0	USD
CY2019	wtm	Members Capital Contribution MembersCapitalContribution	-54600000	USD
CY2018	wtm	Members Capital Contribution MembersCapitalContribution	-53800000	USD
CY2017	wtm	Members Capital Contribution MembersCapitalContribution	-37400000	USD
CY2019	wtm	Fidus Re Premium Payment FidusRePremiumPayment	3000000.0	USD
CY2018	wtm	Fidus Re Premium Payment FidusRePremiumPayment	3700000	USD
CY2017	wtm	Fidus Re Premium Payment FidusRePremiumPayment	0	USD
CY2019	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	-9200000	USD
CY2018	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	-8400000	USD
CY2017	us-gaap	Proceeds From Payments For Other Financing Activities ProceedsFromPaymentsForOtherFinancingActivities	-9300000	USD
CY2019	us-gaap	Net Cash Provided By Used In Financing Activities Continuing Operations NetCashProvidedByUsedInFinancingActivitiesContinuingOperations	230500000	USD
CY2018	us-gaap	Net Cash Provided By Used In Financing Activities Continuing Operations NetCashProvidedByUsedInFinancingActivitiesContinuingOperations	-414300000	USD
CY2017	us-gaap	Net Cash Provided By Used In Financing Activities Continuing Operations NetCashProvidedByUsedInFinancingActivitiesContinuingOperations	-637100000	USD
CY2019	us-gaap	Cash Provided By Used In Financing Activities Discontinued Operations CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations	0	USD
CY2018	us-gaap	Cash Provided By Used In Financing Activities Discontinued Operations CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations	0	USD
CY2017	us-gaap	Cash Provided By Used In Financing Activities Discontinued Operations CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations	-61900000	USD
CY2019	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	230500000	USD
CY2018	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-414300000	USD
CY2017	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-699000000.0	USD
CY2019	us-gaap	Effect Of Exchange Rate On Cash And Cash Equivalents EffectOfExchangeRateOnCashAndCashEquivalents	600000	USD
CY2018	us-gaap	Effect Of Exchange Rate On Cash And Cash Equivalents EffectOfExchangeRateOnCashAndCashEquivalents	-600000	USD
CY2017	us-gaap	Effect Of Exchange Rate On Cash And Cash Equivalents EffectOfExchangeRateOnCashAndCashEquivalents	0	USD
CY2019	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	50700000	USD
CY2018	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	13200000	USD
CY2017	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	16000000.0	USD
CY2018Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	110300000	USD
CY2017Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	97100000	USD
CY2016Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	80200000	USD
CY2018Q4	wtm	Cash Reclassified To Assets Held For Sale CashReclassifiedToAssetsHeldForSale	0	USD
CY2017Q4	wtm	Cash Reclassified To Assets Held For Sale CashReclassifiedToAssetsHeldForSale	0	USD
CY2016Q4	wtm	Cash Reclassified To Assets Held For Sale CashReclassifiedToAssetsHeldForSale	900000	USD
CY2019Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	161000000.0	USD
CY2018Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	110300000	USD
CY2017Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	97100000	USD
CY2019	us-gaap	Use Of Estimates UseOfEstimates	<div style="line-height:120%;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;font-style:italic;font-weight:bold;">Use of Estimates</span></div><div style="line-height:120%;text-indent:24px;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;">The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.</span></div>
CY2019Q4	wtm	Percentage Of Par Value Of Policy Reinsured PercentageOfParValueOfPolicyReinsured	0.15
CY2019Q2	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	81400000	USD
CY2019	wtm	Percentage Of Investments Recorded At Fair Value PercentageOfInvestmentsRecordedAtFairValue	0.71
CY2019	wtm	Fair Value Measurements Variance Thresholdfor Outliers FairValueMeasurementsVarianceThresholdforOutliers	500000	USD
CY2019	wtm	Fair Value Measurements Variance Thresholdfor Outliers Percent FairValueMeasurementsVarianceThresholdforOutliersPercent	0.05
CY2019Q4	us-gaap	Other Long Term Investments OtherLongTermInvestments	856000000	USD
CY2019	us-gaap	Derivatives Reporting Of Derivative Activity DerivativesReportingOfDerivativeActivity	<div style="line-height:120%;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;font-style:italic;font-weight:bold;">Derivatives </span></div><div style="line-height:120%;text-indent:24px;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;">White Mountains holds from time to time a variety of derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either other assets or other liabilities, aside from the foreign currency forward contracts which are recognized within other long-term investments, measured at fair value, in the consolidated balance sheets. Changes in the fair value of derivative instruments that meet the criteria for hedge accounting are recognized in other comprehensive income and reclassified into current period pre-tax income when the hedged items are recognized therein. Changes in the fair value of derivative instruments that do not meet the criteria for hedge accounting are recognized in current period pre-tax income. </span></div><div style="line-height:120%;text-indent:24px;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;">As of December 31, 2019 and 2018, White Mountains holds an interest rate swap derivative instrument that meets the criteria for hedge accounting. In 2018, White Mountains held a foreign currency forward contract derivative instrument that did not meet the criteria for hedge accounting. The foreign currency forward contract was closed in the first quarter of 2018.</span></div><div style="line-height:120%;text-indent:24px;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;">From time to time, White Mountains holds warrants that it has received in the restructuring of certain of its common equity securities and fixed maturity investments. White Mountains accounts for its investments in warrants as derivatives.</span></div>
CY2019	us-gaap	Revenue Recognition Services Commissions RevenueRecognitionServicesCommissions	<div style="line-height:120%;text-align:left;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;font-style:italic;font-weight:bold;">Revenue Recognition</span></div><div style="line-height:120%;text-align:left;text-indent:24px;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;">NSM’s revenues consist primarily of commissions and broker revenues for placement of insurance policies and administrative fees for claims and other services provided to insurance carriers. Commission and broker revenues and service fees are measured based on the contractual rates with insurance carriers, net of any amounts expected to be uncollectible and any amounts associated with expected policy cancellations, adjustments, and are recognized when contractual performance obligations have been fulfilled. NSM’s primary contractual performance obligations are generally satisfied upon the issuance of an insurance policy by the carrier. Where NSM has significant performance obligations beyond the policy issuance date, NSM estimates the relative standalone selling price for the post-issuance services in order to allocate the transaction price using the price charged for the service when sold separately in similar circumstances to similar customers. Deferred revenues associated with unsatisfied performance obligations are recognized within other liabilities.</span></div><div style="line-height:120%;text-align:left;text-indent:24px;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;">Contingent commissions are based upon the overall profit and/or volume of the business placed with the insurance carrier during a calendar year and are determined after the contractual period has ended. NSM recognizes revenue on contingent commissions when management has determined that it is probable that the contingent commission requirements have been met. </span></div><div style="line-height:120%;text-align:left;text-indent:24px;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;">Kudu’s revenues are primarily generated from non-controlling equity interests in revenue and earnings participation contracts with asset management firms. The participation contracts are measured at fair value with the change therein recognized within unrealized investment gains and losses. Distributions from Kudu’s clients are recognized through investment income when Kudu’s right to receive payment has been established and can be reliably measured, which generally occurs on a quarterly basis in accordance with the terms of the underlying participation contracts.</span></div><div style="line-height:120%;text-align:left;text-indent:24px;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;">MediaAlpha recognized advertising and publishing fee revenues based on the contractual amount of the fees, adjusted for any amounts expected to be refunded or uncollectible, when it had satisfied its contractual performance obligations, which was generally at the time each transaction was executed. For transactions where MediaAlpha acted as the principal, such as the Open exchange, revenue amounts were reported gross. For transactions where MediaAlpha acted as an agent facilitating transactions between third parties, revenue amounts were reported at the net fee billed.</span></div><div style="line-height:120%;text-indent:24px;font-size:10pt;"><span style="font-family:inherit;font-size:10pt;"><br/></span></div>
CY2019Q4	us-gaap	Accumulated Other Comprehensive Income Loss Foreign Currency Translation Adjustment Net Of Tax AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax	-2400000	USD
CY2018Q4	us-gaap	Accumulated Other Comprehensive Income Loss Foreign Currency Translation Adjustment Net Of Tax AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax	-3900000	USD
CY2018Q1	us-gaap	Equity Method Investment Aggregate Cost EquityMethodInvestmentAggregateCost	125000000.0	USD
CY2019Q2	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	81400000	USD
CY2019	us-gaap	Interest And Dividend Income Operating InterestAndDividendIncomeOperating	81000000.0	USD
CY2018	us-gaap	Interest And Dividend Income Operating InterestAndDividendIncomeOperating	62000000.0	USD
CY2017	us-gaap	Interest And Dividend Income Operating InterestAndDividendIncomeOperating	58500000	USD
CY2019	us-gaap	Investment Income Investment Expense InvestmentIncomeInvestmentExpense	1300000	USD
CY2018	us-gaap	Investment Income Investment Expense InvestmentIncomeInvestmentExpense	3000000.0	USD
CY2017	us-gaap	Investment Income Investment Expense InvestmentIncomeInvestmentExpense	2500000	USD
CY2019	us-gaap	Net Investment Income NetInvestmentIncome	79700000	USD
CY2018	us-gaap	Net Investment Income NetInvestmentIncome	59000000.0	USD
CY2017	us-gaap	Net Investment Income NetInvestmentIncome	56000000.0	USD
CY2019	wtm	Trading Securities Realized Gain Loss Total TradingSecuritiesRealizedGainLossTotal	96600000	USD
CY2018	wtm	Trading Securities Realized Gain Loss Total TradingSecuritiesRealizedGainLossTotal	-12900000	USD
CY2017	wtm	Trading Securities Realized Gain Loss Total TradingSecuritiesRealizedGainLossTotal	24100000	USD
CY2019	wtm	Net Unrealized Gain Lossand Foreign Currency Gain Losson Trading Securities NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities	336600000	USD
CY2018	wtm	Net Unrealized Gain Lossand Foreign Currency Gain Losson Trading Securities NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities	-95400000	USD
CY2017	wtm	Net Unrealized Gain Lossand Foreign Currency Gain Losson Trading Securities NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities	109200000	USD
CY2019	us-gaap	Net Realized Or Unrealized Gain Loss On Trading Securities NetRealizedOrUnrealizedGainLossOnTradingSecurities	433200000	USD
CY2018	us-gaap	Net Realized Or Unrealized Gain Loss On Trading Securities NetRealizedOrUnrealizedGainLossOnTradingSecurities	-108300000	USD
CY2017	us-gaap	Net Realized Or Unrealized Gain Loss On Trading Securities NetRealizedOrUnrealizedGainLossOnTradingSecurities	133300000	USD
CY2019	wtm	Taxonnetrealizedandunrealizedgainslosstradingsecurities Taxonnetrealizedandunrealizedgainslosstradingsecurities	87000000.0	USD
CY2018	wtm	Taxonnetrealizedandunrealizedgainslosstradingsecurities Taxonnetrealizedandunrealizedgainslosstradingsecurities	-18200000	USD
CY2017	wtm	Taxonnetrealizedandunrealizedgainslosstradingsecurities Taxonnetrealizedandunrealizedgainslosstradingsecurities	12900000	USD
CY2019	wtm	Net Realizedand Unrealized Gain Losson Trading Securities Netof Tax NetRealizedandUnrealizedGainLossonTradingSecuritiesNetofTax	346200000	USD
CY2018	wtm	Net Realizedand Unrealized Gain Losson Trading Securities Netof Tax NetRealizedandUnrealizedGainLossonTradingSecuritiesNetofTax	-90100000	USD
CY2017	wtm	Net Realizedand Unrealized Gain Losson Trading Securities Netof Tax NetRealizedandUnrealizedGainLossonTradingSecuritiesNetofTax	120400000	USD
CY2019	us-gaap	Trading Securities Realized Gain Loss TradingSecuritiesRealizedGainLoss	96600000	USD
CY2019	us-gaap	Foreign Currency Transaction Gain Loss Realized ForeignCurrencyTransactionGainLossRealized	0	USD
CY2019	wtm	Trading Securities Realized Gain Loss Total TradingSecuritiesRealizedGainLossTotal	96600000	USD
CY2019	wtm	Tradingsecurities Taxon Realized Holding Gain Losson Investments TradingsecuritiesTaxonRealizedHoldingGainLossonInvestments	10600000	USD
CY2019	wtm	Trading Securities Taxon Realized Foreign Currency Transaction Gain Loss Tax TradingSecuritiesTaxonRealizedForeignCurrencyTransactionGainLossTax	0	USD
CY2019	wtm	Taxon Net Realized Gain Lossand Foreign Currency Gain Losson Trading Securities TaxonNetRealizedGainLossandForeignCurrencyGainLossonTradingSecurities	10600000	USD
CY2019	wtm	Trading Securities Realized Holding Gain Losson Investmentsafter Tax TradingSecuritiesRealizedHoldingGainLossonInvestmentsafterTax	86000000.0	USD
CY2019	wtm	Trading Securities Realized Foreign Currency Gain Loss Netof Tax TradingSecuritiesRealizedForeignCurrencyGainLossNetofTax	0	USD
CY2019	wtm	Net Realized Gain Lossand Foreign Currency Gain Losson Trading Securities Netof Tax NetRealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesNetofTax	86000000.0	USD
CY2018	us-gaap	Trading Securities Realized Gain Loss TradingSecuritiesRealizedGainLoss	-23900000	USD
CY2018	us-gaap	Foreign Currency Transaction Gain Loss Realized ForeignCurrencyTransactionGainLossRealized	11000000.0	USD
CY2018	wtm	Trading Securities Realized Gain Loss Total TradingSecuritiesRealizedGainLossTotal	-12900000	USD
CY2018	wtm	Tradingsecurities Taxon Realized Holding Gain Losson Investments TradingsecuritiesTaxonRealizedHoldingGainLossonInvestments	-9100000	USD
CY2018	wtm	Trading Securities Taxon Realized Foreign Currency Transaction Gain Loss Tax TradingSecuritiesTaxonRealizedForeignCurrencyTransactionGainLossTax	0	USD
CY2018	wtm	Taxon Net Realized Gain Lossand Foreign Currency Gain Losson Trading Securities TaxonNetRealizedGainLossandForeignCurrencyGainLossonTradingSecurities	-9100000	USD
CY2018	wtm	Trading Securities Realized Holding Gain Losson Investmentsafter Tax TradingSecuritiesRealizedHoldingGainLossonInvestmentsafterTax	-14800000	USD
CY2018	wtm	Trading Securities Realized Foreign Currency Gain Loss Netof Tax TradingSecuritiesRealizedForeignCurrencyGainLossNetofTax	11000000.0	USD
CY2018	wtm	Net Realized Gain Lossand Foreign Currency Gain Losson Trading Securities Netof Tax NetRealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesNetofTax	-3800000	USD
CY2017	us-gaap	Trading Securities Realized Gain Loss TradingSecuritiesRealizedGainLoss	35300000	USD
CY2017	us-gaap	Foreign Currency Transaction Gain Loss Realized ForeignCurrencyTransactionGainLossRealized	-11200000	USD
CY2017	wtm	Trading Securities Realized Gain Loss Total TradingSecuritiesRealizedGainLossTotal	24100000	USD
CY2017	wtm	Tradingsecurities Taxon Realized Holding Gain Losson Investments TradingsecuritiesTaxonRealizedHoldingGainLossonInvestments	-8900000	USD
CY2017	wtm	Trading Securities Taxon Realized Foreign Currency Transaction Gain Loss Tax TradingSecuritiesTaxonRealizedForeignCurrencyTransactionGainLossTax	0	USD
CY2017	wtm	Taxon Net Realized Gain Lossand Foreign Currency Gain Losson Trading Securities TaxonNetRealizedGainLossandForeignCurrencyGainLossonTradingSecurities	-8900000	USD
CY2017	wtm	Trading Securities Realized Holding Gain Losson Investmentsafter Tax TradingSecuritiesRealizedHoldingGainLossonInvestmentsafterTax	26400000	USD
CY2017	wtm	Trading Securities Realized Foreign Currency Gain Loss Netof Tax TradingSecuritiesRealizedForeignCurrencyGainLossNetofTax	-11200000	USD
CY2017	wtm	Net Realized Gain Lossand Foreign Currency Gain Losson Trading Securities Netof Tax NetRealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesNetofTax	15200000	USD
CY2019	us-gaap	Trading Securities Realized Gain TradingSecuritiesRealizedGain	104000000.0	USD
CY2018	us-gaap	Trading Securities Realized Gain TradingSecuritiesRealizedGain	49400000	USD
CY2017	us-gaap	Trading Securities Realized Gain TradingSecuritiesRealizedGain	61500000	USD
CY2019	us-gaap	Trading Securities Realized Loss TradingSecuritiesRealizedLoss	7400000	USD
CY2018	us-gaap	Trading Securities Realized Loss TradingSecuritiesRealizedLoss	62300000	USD
CY2017	us-gaap	Trading Securities Realized Loss TradingSecuritiesRealizedLoss	37400000	USD
CY2019	us-gaap	Trading Securities Unrealized Holding Gain Loss TradingSecuritiesUnrealizedHoldingGainLoss	336900000	USD
CY2019	us-gaap	Foreign Currency Transaction Gain Loss Unrealized ForeignCurrencyTransactionGainLossUnrealized	-300000	USD
CY2019	wtm	Net Unrealized Gain Lossand Foreign Currency Gain Losson Trading Securities NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities	336600000	USD
CY2019	wtm	Trading Securities Taxon Unrealized Holding Gain Losson Investments TradingSecuritiesTaxonUnrealizedHoldingGainLossonInvestments	76400000	USD
CY2019	wtm	Trading Securities Unrealized Foreign Currency Transaction Gain Loss Tax TradingSecuritiesUnrealizedForeignCurrencyTransactionGainLossTax	0	USD
CY2019	wtm	Net Unrealized Gain Lossand Foreign Currency Gain Losson Trading Securities Tax NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesTax	76400000	USD
CY2019	wtm	Trading Securities Unrealized Holding Gain Losson Investmentsafter Tax TradingSecuritiesUnrealizedHoldingGainLossonInvestmentsafterTax	260500000	USD
CY2019	wtm	Trading Securities Unrealized Foreign Currency Gain Loss Net Of Tax TradingSecuritiesUnrealizedForeignCurrencyGainLossNetOfTax	-300000	USD
CY2019	wtm	Net Unrealized Gain Loss And Foreign Currency Gain Loss On Trading Securities Net Of Tax NetUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax	260200000	USD
CY2018	us-gaap	Trading Securities Unrealized Holding Gain Loss TradingSecuritiesUnrealizedHoldingGainLoss	-83600000	USD
CY2018	us-gaap	Foreign Currency Transaction Gain Loss Unrealized ForeignCurrencyTransactionGainLossUnrealized	-11800000	USD
CY2018	wtm	Net Unrealized Gain Lossand Foreign Currency Gain Losson Trading Securities NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities	-95400000	USD
CY2018	wtm	Trading Securities Taxon Unrealized Holding Gain Losson Investments TradingSecuritiesTaxonUnrealizedHoldingGainLossonInvestments	-9100000	USD
CY2018	wtm	Trading Securities Unrealized Foreign Currency Transaction Gain Loss Tax TradingSecuritiesUnrealizedForeignCurrencyTransactionGainLossTax	0	USD
CY2018	wtm	Net Unrealized Gain Lossand Foreign Currency Gain Losson Trading Securities Tax NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesTax	-9100000	USD
CY2018	wtm	Trading Securities Unrealized Holding Gain Losson Investmentsafter Tax TradingSecuritiesUnrealizedHoldingGainLossonInvestmentsafterTax	-74500000	USD
CY2018	wtm	Trading Securities Unrealized Foreign Currency Gain Loss Net Of Tax TradingSecuritiesUnrealizedForeignCurrencyGainLossNetOfTax	-11800000	USD
CY2018	wtm	Net Unrealized Gain Loss And Foreign Currency Gain Loss On Trading Securities Net Of Tax NetUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax	-86300000	USD
CY2017	us-gaap	Trading Securities Unrealized Holding Gain Loss TradingSecuritiesUnrealizedHoldingGainLoss	97500000	USD
CY2017	us-gaap	Foreign Currency Transaction Gain Loss Unrealized ForeignCurrencyTransactionGainLossUnrealized	11700000	USD
CY2017	wtm	Net Unrealized Gain Lossand Foreign Currency Gain Losson Trading Securities NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities	109200000	USD
CY2017	wtm	Trading Securities Taxon Unrealized Holding Gain Losson Investments TradingSecuritiesTaxonUnrealizedHoldingGainLossonInvestments	4000000.0	USD
CY2017	wtm	Trading Securities Unrealized Foreign Currency Transaction Gain Loss Tax TradingSecuritiesUnrealizedForeignCurrencyTransactionGainLossTax	0	USD
CY2017	wtm	Net Unrealized Gain Lossand Foreign Currency Gain Losson Trading Securities Tax NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesTax	4000000.0	USD
CY2017	wtm	Trading Securities Unrealized Holding Gain Losson Investmentsafter Tax TradingSecuritiesUnrealizedHoldingGainLossonInvestmentsafterTax	93500000	USD
CY2017	wtm	Trading Securities Unrealized Foreign Currency Gain Loss Net Of Tax TradingSecuritiesUnrealizedForeignCurrencyGainLossNetOfTax	11700000	USD
CY2017	wtm	Net Unrealized Gain Loss And Foreign Currency Gain Loss On Trading Securities Net Of Tax NetUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax	105200000	USD
CY2019Q4	wtm	Trading Securities Amortized Cost TradingSecuritiesAmortizedCost	1177900000	USD
CY2019Q4	us-gaap	Trading Securities TradingSecurities	1205800000	USD
CY2019	wtm	Proceedsfrom Saleand Maturityof Trading Securities Heldforinvestment Excluding Short Term Securities ProceedsfromSaleandMaturityofTradingSecuritiesHeldforinvestmentExcludingShortTermSecurities	1000000000.0	USD
CY2018	wtm	Proceedsfrom Saleand Maturityof Trading Securities Heldforinvestment Excluding Short Term Securities ProceedsfromSaleandMaturityofTradingSecuritiesHeldforinvestmentExcludingShortTermSecurities	2200000000	USD
CY2017	wtm	Proceedsfrom Saleand Maturityof Trading Securities Heldforinvestment Excluding Short Term Securities ProceedsfromSaleandMaturityofTradingSecuritiesHeldforinvestmentExcludingShortTermSecurities	2800000000	USD
CY2019Q4	wtm	Investments Held In Trusts InvestmentsHeldInTrusts	319900000	USD
CY2018Q4	wtm	Investments Held In Trusts InvestmentsHeldInTrusts	254300000	USD
CY2019Q4	us-gaap	Assets Held By Insurance Regulators AssetsHeldByInsuranceRegulators	9900000	USD
CY2018Q4	us-gaap	Assets Held By Insurance Regulators AssetsHeldByInsuranceRegulators	6100000	USD
CY2019	wtm	Percentage Of Investments Recorded At Fair Value PercentageOfInvestmentsRecordedAtFairValue	0.71
CY2018	wtm	Percentage Of Investments Recorded At Fair Value PercentageOfInvestmentsRecordedAtFairValue	0.87
CY2019Q4	us-gaap	Other Investments OtherInvestments	622800000	USD
CY2018Q4	us-gaap	Other Investments OtherInvestments	165700000	USD
CY2018Q4	us-gaap	Investments Fair Value Disclosure InvestmentsFairValueDisclosure	2328700000	USD
CY2019	wtm	Fair Value Assets Measured On Recurring Basis Gain Loss Included In Earnings FairValueAssetsMeasuredOnRecurringBasisGainLossIncludedInEarnings	-433000000.0	USD
CY2019	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	1600000	USD
CY2019	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Purchases FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases	866100000	USD
CY2019	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Sales FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales	950300000	USD
CY2019	wtm	Fair Value Assets Measured On Recurring Basis Transfers In FairValueAssetsMeasuredOnRecurringBasisTransfersIn	14900000	USD
CY2019	wtm	Fair Value Assets Measured On Recurring Basis Transfers Out FairValueAssetsMeasuredOnRecurringBasisTransfersOut	14900000	USD
CY2019Q4	us-gaap	Investments Fair Value Disclosure InvestmentsFairValueDisclosure	2746000000.0	USD
CY2017Q4	us-gaap	Investments Fair Value Disclosure InvestmentsFairValueDisclosure	3208300000	USD
CY2018	wtm	Fair Value Assets Measured On Recurring Basis Gain Loss Included In Earnings FairValueAssetsMeasuredOnRecurringBasisGainLossIncludedInEarnings	99800000	USD
CY2018	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	2500000	USD
CY2018	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Purchases FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases	1394300000	USD
CY2018	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Sales FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales	2171600000	USD
CY2018	wtm	Fair Value Assets Measured On Recurring Basis Transfers In FairValueAssetsMeasuredOnRecurringBasisTransfersIn	0	USD
CY2018	wtm	Fair Value Assets Measured On Recurring Basis Transfers Out FairValueAssetsMeasuredOnRecurringBasisTransfersOut	0	USD
CY2019Q4	us-gaap	Goodwill Fair Value Disclosure GoodwillFairValueDisclosure	402300000	USD
CY2019	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	7600000	USD
CY2019Q4	us-gaap	Goodwill Goodwill	394700000	USD
CY2018Q4	us-gaap	Goodwill Fair Value Disclosure GoodwillFairValueDisclosure	379900000	USD
CY2018Q4	us-gaap	Goodwill Goodwill	379900000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Fair Value Disclosure FiniteLivedIntangibleAssetsFairValueDisclosure	694400000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	29300000	USD
CY2019	us-gaap	Goodwill And Intangible Asset Impairment GoodwillAndIntangibleAssetImpairment	10400000	USD
CY2019Q4	us-gaap	Intangible Assets Net Including Goodwill IntangibleAssetsNetIncludingGoodwill	654700000	USD
CY2018Q4	us-gaap	Finite Lived Intangible Assets Fair Value Disclosure FiniteLivedIntangibleAssetsFairValueDisclosure	591100000	USD
CY2018Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	53600000	USD
CY2018Q4	us-gaap	Intangible Assets Net Including Goodwill IntangibleAssetsNetIncludingGoodwill	537500000	USD
CY2019	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	7600000	USD
CY2019	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	21800000	USD
CY2018	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	18800000	USD
CY2017	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	10700000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Next Twelve Months FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths	23900000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Two FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo	30500000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Three FiniteLivedIntangibleAssetsAmortizationExpenseYearThree	29700000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Four FiniteLivedIntangibleAssetsAmortizationExpenseYearFour	28400000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Five FiniteLivedIntangibleAssetsAmortizationExpenseYearFive	138900000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Net FiniteLivedIntangibleAssetsNet	251400000	USD
CY2019Q4	us-gaap	Indefinite Lived Intangible Assets Excluding Goodwill IndefiniteLivedIntangibleAssetsExcludingGoodwill	8600000	USD
CY2019Q4	us-gaap	Long Term Debt LongTermDebt	283500000	USD
CY2018Q4	us-gaap	Long Term Debt LongTermDebt	192700000	USD
CY2019Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal After Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive	55200000	USD
CY2019Q4	wtm	Long Term Debt Maturities Repayments Of Principal Total LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	291200000	USD
CY2019	us-gaap	Interest Expense Debt InterestExpenseDebt	17600000	USD
CY2018	us-gaap	Interest Expense Debt InterestExpenseDebt	9500000	USD
CY2017	us-gaap	Interest Expense Debt InterestExpenseDebt	2300000	USD
CY2019Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	2800000	USD
CY2019Q4	us-gaap	Long Term Debt Maturing In Years Two And Three LongTermDebtMaturingInYearsTwoAndThree	7000000.0	USD
CY2019Q4	us-gaap	Long Term Debt Maturing In Years Four And Five LongTermDebtMaturingInYearsFourAndFive	226200000	USD
CY2019	us-gaap	Interest Paid InterestPaid	16300000	USD
CY2018	us-gaap	Interest Paid InterestPaid	8800000	USD
CY2017	us-gaap	Interest Paid InterestPaid	1400000	USD
CY2019	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	-900000	USD
CY2018	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	100000	USD
CY2017	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	300000	USD
CY2019	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	3700000	USD
CY2018	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	1400000	USD
CY2017	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	1300000	USD
CY2019	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	1700000	USD
CY2018	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	2900000	USD
CY2017	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	2000000.0	USD
CY2019	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	4500000	USD
CY2018	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	4400000	USD
CY2017	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	3600000	USD
CY2019	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	14900000	USD
CY2018	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-8300000	USD
CY2017	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-11400000	USD
CY2019	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-10400000	USD
CY2018	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	0	USD
CY2017	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	0	USD
CY2019	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	-500000	USD
CY2019	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.21
CY2018	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.35
CY2019	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	85100000	USD
CY2018	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	-37400000	USD
CY2017	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	2700000	USD
CY2019	us-gaap	Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	-63600000	USD
CY2018	us-gaap	Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	31000000.0	USD
CY2017	us-gaap	Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	-42600000	USD
CY2019	us-gaap	Income Tax Reconciliation Foreign Income Tax Rate Differential IncomeTaxReconciliationForeignIncomeTaxRateDifferential	-27100000	USD
CY2018	us-gaap	Income Tax Reconciliation Foreign Income Tax Rate Differential IncomeTaxReconciliationForeignIncomeTaxRateDifferential	2900000	USD
CY2017	us-gaap	Income Tax Reconciliation Foreign Income Tax Rate Differential IncomeTaxReconciliationForeignIncomeTaxRateDifferential	-21500000	USD
CY2018	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	-100000	USD
CY2017	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	0	USD
CY2019	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	24800000	USD
CY2018	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-8400000	USD
CY2017	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-11400000	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	29300000	USD
CY2018	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	-4000000.0	USD
CY2017	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	-7800000	USD
CY2019	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	17500000	USD
CY2018	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	-4000000.0	USD
CY2017	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	-600000	USD
CY2019	us-gaap	Income Tax Reconciliation Change In Enacted Tax Rate IncomeTaxReconciliationChangeInEnactedTaxRate	5700000	USD
CY2018	us-gaap	Income Tax Reconciliation Change In Enacted Tax Rate IncomeTaxReconciliationChangeInEnactedTaxRate	-1700000	USD
CY2017	us-gaap	Income Tax Reconciliation Change In Enacted Tax Rate IncomeTaxReconciliationChangeInEnactedTaxRate	44300000	USD
CY2019	wtm	Effective Income Tax Rate Reconciliation Member Surplus Contribution Amount EffectiveIncomeTaxRateReconciliationMemberSurplusContributionAmount	3600000	USD
CY2018	wtm	Effective Income Tax Rate Reconciliation Member Surplus Contribution Amount EffectiveIncomeTaxRateReconciliationMemberSurplusContributionAmount	2600000	USD
CY2017	wtm	Effective Income Tax Rate Reconciliation Member Surplus Contribution Amount EffectiveIncomeTaxRateReconciliationMemberSurplusContributionAmount	3000000.0	USD
CY2019	wtm	Effective Income Tax Rate Reduction Withholding Tax Amount EffectiveIncomeTaxRateReductionWithholdingTaxAmount	1600000	USD
CY2018	wtm	Effective Income Tax Rate Reduction Withholding Tax Amount EffectiveIncomeTaxRateReductionWithholdingTaxAmount	2700000	USD
CY2017	wtm	Effective Income Tax Rate Reduction Withholding Tax Amount EffectiveIncomeTaxRateReductionWithholdingTaxAmount	2000000.0	USD
CY2019	us-gaap	Income Tax Reconciliation Tax Exempt Income IncomeTaxReconciliationTaxExemptIncome	1100000	USD
CY2018	us-gaap	Income Tax Reconciliation Tax Exempt Income IncomeTaxReconciliationTaxExemptIncome	600000	USD
CY2017	us-gaap	Income Tax Reconciliation Tax Exempt Income IncomeTaxReconciliationTaxExemptIncome	500000	USD
CY2019	us-gaap	Income Tax Reconciliation Tax Contingencies IncomeTaxReconciliationTaxContingencies	700000	USD
CY2018	us-gaap	Income Tax Reconciliation Tax Contingencies IncomeTaxReconciliationTaxContingencies	800000	USD
CY2017	us-gaap	Income Tax Reconciliation Tax Contingencies IncomeTaxReconciliationTaxContingencies	300000	USD
CY2019	us-gaap	Income Tax Reconciliation Nondeductible Expense Share Based Compensation Cost IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost	0	USD
CY2018	us-gaap	Income Tax Reconciliation Nondeductible Expense Share Based Compensation Cost IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost	0	USD
CY2017	us-gaap	Income Tax Reconciliation Nondeductible Expense Share Based Compensation Cost IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost	4100000	USD
CY2019	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	6900000	USD
CY2018	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	-300000	USD
CY2017	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	1000000.0	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	29300000	USD
CY2018	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	-4000000.0	USD
CY2017	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	-7800000	USD
CY2019	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	3700000	USD
CY2018	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	3500000	USD
CY2017	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	2000000.0	USD
CY2019Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards Domestic DeferredTaxAssetsOperatingLossCarryforwardsDomestic	92300000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards Domestic DeferredTaxAssetsOperatingLossCarryforwardsDomestic	95500000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards Foreign DeferredTaxAssetsOperatingLossCarryforwardsForeign	41700000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards Foreign DeferredTaxAssetsOperatingLossCarryforwardsForeign	36600000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits	15000000.0	USD
CY2018Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits	14100000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Investments DeferredTaxAssetsInvestments	0	USD
CY2018Q4	us-gaap	Deferred Tax Assets Investments DeferredTaxAssetsInvestments	11100000	USD
CY2019Q4	wtm	Deferred Tax Assets Unrealized Investment Gains Net DeferredTaxAssetsUnrealizedInvestmentGainsNet	0	USD
CY2018Q4	wtm	Deferred Tax Assets Unrealized Investment Gains Net DeferredTaxAssetsUnrealizedInvestmentGainsNet	9500000	USD
CY2019Q4	wtm	Deferred Tax Asset Accrued Interest DeferredTaxAssetAccruedInterest	5100000	USD
CY2018Q4	wtm	Deferred Tax Asset Accrued Interest DeferredTaxAssetAccruedInterest	0	USD
CY2019Q4	wtm	Deferred Tax Assets Deferred Expense Deferred Policy Acquisition Cost DeferredTaxAssetsDeferredExpenseDeferredPolicyAcquisitionCost	4300000	USD
CY2018Q4	wtm	Deferred Tax Assets Deferred Expense Deferred Policy Acquisition Cost DeferredTaxAssetsDeferredExpenseDeferredPolicyAcquisitionCost	3500000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards Other DeferredTaxAssetsTaxCreditCarryforwardsOther	4200000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards Other DeferredTaxAssetsTaxCreditCarryforwardsOther	4200000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	3400000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	5800000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	166000000.0	USD
CY2018Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	180300000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	77900000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	139900000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	88100000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	40400000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Unrealized Gains On Trading Securities DeferredTaxLiabilitiesUnrealizedGainsOnTradingSecurities	66300000	USD
CY2018Q4	us-gaap	Deferred Tax Liabilities Unrealized Gains On Trading Securities DeferredTaxLiabilitiesUnrealizedGainsOnTradingSecurities	0	USD
CY2019Q4	wtm	Deferredtaxliabilitiesmemberssurpluscontributions Deferredtaxliabilitiesmemberssurpluscontributions	43200000	USD
CY2018Q4	wtm	Deferredtaxliabilitiesmemberssurpluscontributions Deferredtaxliabilitiesmemberssurpluscontributions	32800000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Investments DeferredTaxLiabilitiesInvestments	6600000	USD
CY2018Q4	us-gaap	Deferred Tax Liabilities Investments DeferredTaxLiabilitiesInvestments	0	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Other DeferredTaxLiabilitiesOther	4600000	USD
CY2018Q4	us-gaap	Deferred Tax Liabilities Other DeferredTaxLiabilitiesOther	5400000	USD
CY2019Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	120700000	USD
CY2018Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	38200000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	32600000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	2200000	USD
CY2017Q4	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.35
CY2017Q4	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.21
CY2018	wtm	Percentageofutilizationofnetoperatinglosses Percentageofutilizationofnetoperatinglosses	0.80
CY2019Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	77900000	USD
CY2018Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	139900000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards Foreign DeferredTaxAssetsTaxCreditCarryforwardsForeign	3800000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards Alternative Minimum Tax DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax	400000	USD
CY2017	wtm	Percentage Of Likelihood For Realization Of Benefit Upon Ultimate Settlement PercentageOfLikelihoodForRealizationOfBenefitUponUltimateSettlement	0.50
CY2016Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	0	USD
CY2017	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	100000	USD
CY2017	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions	200000	USD
CY2017Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	300000	USD
CY2018	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	800000	USD
CY2018Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	1100000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	1300000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	2400000	USD
CY2018Q4	wtm	Percentageof Risk Premium Ceded PercentageofRiskPremiumCeded	0.60
CY2019Q4	us-gaap	Assets Held In Trust AssetsHeldInTrust	786700000	USD
CY2018Q4	us-gaap	Assets Held In Trust AssetsHeldInTrust	757400000	USD
CY2019Q4	us-gaap	Interest Receivable InterestReceivable	162700000	USD
CY2018Q4	us-gaap	Interest Receivable InterestReceivable	143700000	USD
CY2018	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	3342400	shares
CY2017	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	4239500	shares
CY2019	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	414500000	USD
CY2018	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-141200000	USD
CY2017	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	627200000	USD
CY2019	us-gaap	Income Loss From Discontinued Operations Net Of Tax Attributable To Reporting Entity IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity	800000	USD
CY2018	us-gaap	Income Loss From Discontinued Operations Net Of Tax Attributable To Reporting Entity IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity	-17200000	USD
CY2017	us-gaap	Income Loss From Discontinued Operations Net Of Tax Attributable To Reporting Entity IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity	577500000	USD
CY2019	us-gaap	Income Loss From Continuing Operations IncomeLossFromContinuingOperations	413700000	USD
CY2018	us-gaap	Income Loss From Continuing Operations IncomeLossFromContinuingOperations	-124000000.0	USD
CY2017	us-gaap	Income Loss From Continuing Operations IncomeLossFromContinuingOperations	49700000	USD
CY2019	wtm	Income Loss Allocated To Participating Unvested Restricted Common Shares IncomeLossAllocatedToParticipatingUnvestedRestrictedCommonShares	5300000	USD
CY2018	wtm	Income Loss Allocated To Participating Unvested Restricted Common Shares IncomeLossAllocatedToParticipatingUnvestedRestrictedCommonShares	-1400000	USD
CY2017	wtm	Income Loss Allocated To Participating Unvested Restricted Common Shares IncomeLossAllocatedToParticipatingUnvestedRestrictedCommonShares	700000	USD
CY2019	wtm	Net Income Loss Attributableto Parent Basicand Diluted Earnings Per Share Numerators NetIncomeLossAttributabletoParentBasicandDilutedEarningsPerShareNumerators	408400000	USD
CY2018	wtm	Net Income Loss Attributableto Parent Basicand Diluted Earnings Per Share Numerators NetIncomeLossAttributabletoParentBasicandDilutedEarningsPerShareNumerators	-122600000	USD
CY2017	wtm	Net Income Loss Attributableto Parent Basicand Diluted Earnings Per Share Numerators NetIncomeLossAttributabletoParentBasicandDilutedEarningsPerShareNumerators	49000000.0	USD
CY2019	us-gaap	Weighted Average Number Of Shares Issued Basic WeightedAverageNumberOfSharesIssuedBasic	3181600	shares
CY2018	us-gaap	Weighted Average Number Of Shares Issued Basic WeightedAverageNumberOfSharesIssuedBasic	3382500	shares
CY2017	us-gaap	Weighted Average Number Of Shares Issued Basic WeightedAverageNumberOfSharesIssuedBasic	4293800	shares
CY2019	us-gaap	Weighted Average Number Of Shares Restricted Stock WeightedAverageNumberOfSharesRestrictedStock	40500	shares
CY2018	us-gaap	Weighted Average Number Of Shares Restricted Stock WeightedAverageNumberOfSharesRestrictedStock	40100	shares
CY2017	us-gaap	Weighted Average Number Of Shares Restricted Stock WeightedAverageNumberOfSharesRestrictedStock	54300	shares
CY2019	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	3141100	shares
CY2018	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	3342400	shares
CY2017	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	4239500	shares
CY2019	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	3181600	shares
CY2018	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	3382500	shares
CY2017	us-gaap	Weighted Average Number Diluted Shares Outstanding Adjustment WeightedAverageNumberDilutedSharesOutstandingAdjustment	4293800	shares
CY2019	us-gaap	Weighted Average Number Of Shares Restricted Stock WeightedAverageNumberOfSharesRestrictedStock	40500	shares
CY2018	us-gaap	Weighted Average Number Of Shares Restricted Stock WeightedAverageNumberOfSharesRestrictedStock	40100	shares
CY2017	us-gaap	Weighted Average Number Of Shares Restricted Stock WeightedAverageNumberOfSharesRestrictedStock	54300	shares
CY2019	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	3141100	shares
CY2017	wtm	Dividends Common Stock Cash Net Of Restricted Common Share Amounts DividendsCommonStockCashNetOfRestrictedCommonShareAmounts	4500000	USD
CY2019	us-gaap	Undistributed Earnings UndistributedEarnings	405200000	USD
CY2019	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.00
CY2018	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.00
CY2017	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.00
CY2019	us-gaap	Earnings Per Share Diluted Undistributed EarningsPerShareDilutedUndistributed	129.02
CY2018	us-gaap	Earnings Per Share Diluted Undistributed EarningsPerShareDilutedUndistributed	-37.67
CY2017	us-gaap	Earnings Per Share Diluted Undistributed EarningsPerShareDilutedUndistributed	10.56
CY2019	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	130.02
CY2018	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	-36.67
CY2017	us-gaap	Earnings Per Share Basic And Diluted EarningsPerShareBasicAndDiluted	11.56
CY2019	wtm	Income Loss From Continuing Operatings Excluding Portion Attributable To Noncontrolling Interest Net IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet	408400000	USD
CY2018	wtm	Income Loss From Continuing Operatings Excluding Portion Attributable To Noncontrolling Interest Net IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet	-122600000	USD
CY2017	wtm	Income Loss From Continuing Operatings Excluding Portion Attributable To Noncontrolling Interest Net IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet	49000000.0	USD
CY2019	wtm	Dividends Common Stock Cash Net Of Restricted Common Share Amounts DividendsCommonStockCashNetOfRestrictedCommonShareAmounts	3200000	USD
CY2018	wtm	Dividends Common Stock Cash Net Of Restricted Common Share Amounts DividendsCommonStockCashNetOfRestrictedCommonShareAmounts	3700000	USD
CY2018	us-gaap	Undistributed Earnings UndistributedEarnings	-126300000	USD
CY2017	us-gaap	Undistributed Earnings UndistributedEarnings	44500000	USD
CY2017Q4	us-gaap	Shares Issued Price Per Share SharesIssuedPricePerShare	742
CY2019	us-gaap	Operating Lease Cost OperatingLeaseCost	7200000	USD
CY2019	us-gaap	Sublease Income SubleaseIncome	400000	USD
CY2019	us-gaap	Lease Cost LeaseCost	6800000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Remainder Of Fiscal Year LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear	5900000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Two LesseeOperatingLeaseLiabilityPaymentsDueYearTwo	4900000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Three LesseeOperatingLeaseLiabilityPaymentsDueYearThree	4500000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Four LesseeOperatingLeaseLiabilityPaymentsDueYearFour	3900000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Five LesseeOperatingLeaseLiabilityPaymentsDueYearFive	3100000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due After Year Five LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive	3400000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	25700000	USD
CY2019Q4	us-gaap	Lessee Operating Lease Liability Undiscounted Excess Amount LesseeOperatingLeaseLiabilityUndiscountedExcessAmount	2900000	USD
CY2019Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	22800000	USD
CY2019Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	22600000	USD
CY2019Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.046
CY2019Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	22800000	USD
CY2019	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	4900000	USD
CY2019	us-gaap	Treasury Stock Acquired Average Cost Per Share TreasuryStockAcquiredAverageCostPerShare	858
CY2018	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	519100000	USD
CY2018	us-gaap	Treasury Stock Acquired Average Cost Per Share TreasuryStockAcquiredAverageCostPerShare	877
CY2017	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	723900000	USD
CY2017	us-gaap	Treasury Stock Acquired Average Cost Per Share TreasuryStockAcquiredAverageCostPerShare	869
CY2019	us-gaap	Stock Issued During Period Shares New Issues StockIssuedDuringPeriodSharesNewIssues	17917	shares
CY2019	wtm	Stock Issued During Period To Directors StockIssuedDuringPeriodToDirectors	2317	shares
CY2018	us-gaap	Stock Issued During Period Shares New Issues StockIssuedDuringPeriodSharesNewIssues	16377	shares
CY2018	wtm	Stock Issued During Period To Directors StockIssuedDuringPeriodToDirectors	2272	shares
CY2017	us-gaap	Stock Issued During Period Shares New Issues StockIssuedDuringPeriodSharesNewIssues	25086	shares
CY2019	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	3200000	USD
CY2018	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	3800000	USD
CY2017	us-gaap	Premiums Earned Net PremiumsEarnedNet	10400000	USD
CY2017	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	4600000	USD
CY2019	us-gaap	Common Stock Dividends Per Share Cash Paid CommonStockDividendsPerShareCashPaid	1.00
CY2019Q4	us-gaap	Minority Interest MinorityInterest	-116800000	USD
CY2018Q4	us-gaap	Minority Interest Ownership Percentage By Noncontrolling Owners MinorityInterestOwnershipPercentageByNoncontrollingOwners
CY2018Q4	us-gaap	Minority Interest MinorityInterest	-124900000	USD
CY2019	us-gaap	Premiums Earned Net PremiumsEarnedNet	16300000	USD
CY2019	us-gaap	Net Investment Income NetInvestmentIncome	79700000	USD
CY2019	wtm	Gain Loss On Investments Net Of Tax GainLossOnInvestmentsNetOfTax	318500000	USD
CY2019	wtm	Commission Revenue CommissionRevenue	249100000	USD
CY2019	us-gaap	Other Income OtherIncome	47600000	USD
CY2019	us-gaap	Revenues Revenues	893400000	USD
CY2019	us-gaap	Deferred Policy Acquisition Cost Amortization Expense DeferredPolicyAcquisitionCostAmortizationExpense	5700000	USD
CY2019	us-gaap	Other Underwriting Expense OtherUnderwritingExpense	400000	USD
CY2019	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	48100000	USD
CY2019	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	327800000	USD
CY2019	wtm	Broker Commission Expense BrokerCommissionExpense	64800000	USD
CY2019	us-gaap	Business Combination Contingent Consideration Arrangements Change In Amount Of Contingent Consideration Liability1 BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1	2100000	USD
CY2019	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	21800000	USD
CY2019	us-gaap	Interest Expense Debt InterestExpenseDebt	17600000	USD
CY2019	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	488300000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	405100000	USD
CY2018	us-gaap	Premiums Earned Net PremiumsEarnedNet	13900000	USD
CY2018	us-gaap	Net Investment Income NetInvestmentIncome	59000000.0	USD
CY2018	us-gaap	Gain Loss On Investments GainLossOnInvestments	-108300000	USD
CY2018	wtm	Commission Revenue CommissionRevenue	389200000	USD
CY2018	us-gaap	Other Income OtherIncome	15300000	USD
CY2018	us-gaap	Revenues Revenues	369100000	USD
CY2018	us-gaap	Deferred Policy Acquisition Cost Amortization Expense DeferredPolicyAcquisitionCostAmortizationExpense	5300000	USD
CY2018	us-gaap	Other Underwriting Expense OtherUnderwritingExpense	400000	USD
CY2018	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	248700000	USD
CY2018	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	233500000	USD
CY2018	wtm	Broker Commission Expense BrokerCommissionExpense	28400000	USD
CY2018	us-gaap	Business Combination Contingent Consideration Arrangements Change In Amount Of Contingent Consideration Liability1 BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1	2700000	USD
CY2018	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	18800000	USD
CY2018	us-gaap	Interest Expense Debt InterestExpenseDebt	9500000	USD
CY2018	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	547300000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	-178200000	USD
CY2019Q4	us-gaap	Minority Interest MinorityInterest	-116800000	USD
CY2017	us-gaap	Net Investment Income NetInvestmentIncome	56000000.0	USD
CY2017	us-gaap	Gain Loss On Investments GainLossOnInvestments	133300000	USD
CY2017	wtm	Commission Revenue CommissionRevenue	167000000.0	USD
CY2017	us-gaap	Other Income OtherIncome	7100000	USD
CY2017	us-gaap	Revenues Revenues	373800000	USD
CY2017	us-gaap	Policyholder Benefits And Claims Incurred Net PolicyholderBenefitsAndClaimsIncurredNet	1100000	USD
CY2017	us-gaap	Deferred Policy Acquisition Cost Amortization Expense DeferredPolicyAcquisitionCostAmortizationExpense	4100000	USD
CY2017	us-gaap	Other Underwriting Expense OtherUnderwritingExpense	400000	USD
CY2017	us-gaap	Cost Of Goods And Services Sold CostOfGoodsAndServicesSold	139400000	USD
CY2017	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	208000000.0	USD
CY2017	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	10700000	USD
CY2017	us-gaap	Interest Expense Debt InterestExpenseDebt	2300000	USD
CY2017	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	366000000.0	USD
CY2017	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	7800000	USD
CY2019Q4	us-gaap	Investments Investments	2947200000	USD
CY2019Q4	us-gaap	Assets Assets	3983200000	USD
CY2019Q4	us-gaap	Liabilities Liabilities	838500000	USD
CY2019Q4	us-gaap	Stockholders Equity StockholdersEquity	3261500000	USD
CY2018Q4	us-gaap	Investments Investments	2542900000	USD
CY2018Q4	us-gaap	Assets Assets	3362600000	USD
CY2018Q4	us-gaap	Liabilities Liabilities	644400000	USD
CY2018Q4	us-gaap	Stockholders Equity StockholdersEquity	2843100000	USD
CY2018Q4	us-gaap	Minority Interest MinorityInterest	-124900000	USD
CY2019	us-gaap	Revenue From Contract With Customer Including Assessed Tax RevenueFromContractWithCustomerIncludingAssessedTax	294300000	USD
CY2019	wtm	Other Revenue OtherRevenue	599100000	USD
CY2019	us-gaap	Revenues Revenues	893400000	USD
CY2018	us-gaap	Revenue From Contract With Customer Including Assessed Tax RevenueFromContractWithCustomerIncludingAssessedTax	401200000	USD
CY2018	wtm	Other Revenue OtherRevenue	-32100000	USD
CY2018	us-gaap	Revenues Revenues	369100000	USD
CY2017	us-gaap	Revenue From Contract With Customer Including Assessed Tax RevenueFromContractWithCustomerIncludingAssessedTax	163600000	USD
CY2017	wtm	Other Revenue OtherRevenue	210200000	USD
CY2017	us-gaap	Revenues Revenues	373800000	USD
CY2019Q2	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	81400000	USD
CY2019Q2	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	113200000	USD
CY2019Q1	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	151900000	USD
CY2018Q4	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	150100000	USD
CY2019	us-gaap	Operating Leases Rent Expense Net OperatingLeasesRentExpenseNet	7200000	USD
CY2018	us-gaap	Operating Leases Rent Expense Net OperatingLeasesRentExpenseNet	5500000	USD
CY2017	us-gaap	Operating Leases Rent Expense Net OperatingLeasesRentExpenseNet	3500000	USD
CY2019Q4	us-gaap	Capital Leases Future Minimum Payments Due Current CapitalLeasesFutureMinimumPaymentsDueCurrent	7100000	USD
CY2019Q4	us-gaap	Capital Leases Future Minimum Payments Due In Two Years CapitalLeasesFutureMinimumPaymentsDueInTwoYears	6100000	USD
CY2019Q4	us-gaap	Capital Leases Future Minimum Payments Due In Three Years CapitalLeasesFutureMinimumPaymentsDueInThreeYears	5700000	USD
CY2019Q4	us-gaap	Capital Leases Future Minimum Payments Due In Four Years CapitalLeasesFutureMinimumPaymentsDueInFourYears	18400000	USD
CY2019	wtm	Commitments Other Long Term Investments CommitmentsOtherLongTermInvestments	183400000	USD
CY2017	us-gaap	Cash Provided By Used In Operating Activities Discontinued Operations CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations	157000000.0	USD
CY2017	us-gaap	Cash Provided By Used In Investing Activities Discontinued Operations CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations	3000000.0	USD
CY2017	us-gaap	Cash Provided By Used In Financing Activities Discontinued Operations CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations	-61900000	USD
CY2017	us-gaap	Net Cash Provided By Used In Discontinued Operations NetCashProvidedByUsedInDiscontinuedOperations	98100000	USD
CY2017	us-gaap	Cash And Cash Equivalents Period Increase Decrease CashAndCashEquivalentsPeriodIncreaseDecrease	-900000	USD
CY2017	us-gaap	Cash Divested From Deconsolidation CashDivestedFromDeconsolidation	167700000	USD
CY2019Q4	us-gaap	Revenues Revenues	175300000	USD
CY2019Q3	us-gaap	Revenues Revenues	155500000	USD
CY2019Q2	us-gaap	Revenues Revenues	129000000.0	USD
CY2019Q1	us-gaap	Revenues Revenues	433600000	USD
CY2018Q4	us-gaap	Revenues Revenues	6000000.0	USD
CY2018Q3	us-gaap	Revenues Revenues	198700000	USD
CY2018Q2	us-gaap	Revenues Revenues	122300000	USD
CY2018Q1	us-gaap	Revenues Revenues	42100000	USD
CY2019Q4	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	118800000	USD
CY2019Q3	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	104400000	USD
CY2018Q3	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	154400000	USD
CY2018Q2	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	134700000	USD
CY2018Q1	us-gaap	Benefits Losses And Expenses BenefitsLossesAndExpenses	108100000	USD
CY2019Q4	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	56500000	USD
CY2019Q3	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	51100000	USD
CY2019Q2	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	15800000	USD
CY2019Q1	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	281700000	USD
CY2018Q4	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	-144100000	USD
CY2018Q3	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	44300000	USD
CY2018Q2	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	-12400000	USD
CY2018Q1	us-gaap	Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	-66000000.0	USD
CY2019Q4	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	10400000	USD
CY2019Q3	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	8800000	USD
CY2019Q2	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	-100000	USD
CY2019Q1	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	10200000	USD
CY2018Q4	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	-3600000	USD
CY2018Q3	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	-3600000	USD
CY2018Q2	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	2500000	USD
CY2018Q1	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	700000	USD
CY2019Q4	us-gaap	Income Loss From Continuing Operations Including Portion Attributable To Noncontrolling Interest IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest	46100000	USD
CY2019Q3	us-gaap	Income Loss From Continuing Operations Including Portion Attributable To Noncontrolling Interest IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest	42300000	USD
CY2019Q2	us-gaap	Income Loss From Continuing Operations Including Portion Attributable To Noncontrolling Interest IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest	15900000	USD
CY2019Q1	us-gaap	Income Loss From Continuing Operations Including Portion Attributable To Noncontrolling Interest IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest	271500000	USD
CY2018Q4	us-gaap	Income Loss From Continuing Operations Including Portion Attributable To Noncontrolling Interest IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest	-140500000	USD
CY2018Q3	us-gaap	Income Loss From Continuing Operations Including Portion Attributable To Noncontrolling Interest IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest	47900000	USD
CY2018Q2	us-gaap	Income Loss From Continuing Operations Including Portion Attributable To Noncontrolling Interest IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest	-14900000	USD
CY2018Q1	us-gaap	Income Loss From Continuing Operations Including Portion Attributable To Noncontrolling Interest IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest	-66700000	USD
CY2019Q4	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	-800000	USD
CY2019Q3	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	900000	USD
CY2019Q2	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	0	USD
CY2019Q1	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	700000	USD
CY2018Q4	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	0	USD
CY2018Q3	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	-17300000	USD
CY2018Q2	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	0	USD
CY2018Q1	us-gaap	Income Loss From Discontinued Operations Net Of Tax IncomeLossFromDiscontinuedOperationsNetOfTax	100000	USD
CY2019Q4	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	-15600000	USD
CY2019Q3	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	-5500000	USD
CY2019Q2	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	-4600000	USD
CY2019Q1	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	-12200000	USD
CY2018Q4	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	-3000000.0	USD
CY2018Q3	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	-10200000	USD
CY2018Q2	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	-18400000	USD
CY2018Q1	us-gaap	Net Income Loss Attributable To Noncontrolling Interest NetIncomeLossAttributableToNoncontrollingInterest	-18600000	USD
CY2019Q4	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	60900000	USD
CY2019Q3	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	48700000	USD
CY2019Q2	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	20500000	USD
CY2019Q1	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	284400000	USD
CY2018Q4	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-137500000	USD
CY2018Q3	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	40800000	USD
CY2018Q2	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	3500000	USD
CY2018Q1	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-48000000.0	USD
CY2019Q4	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	19.37
CY2019Q3	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	15.01
CY2019Q2	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	6.44
CY2019Q1	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	89.42
CY2018Q4	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	-43.24
CY2018Q3	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	18.27
CY2018Q2	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	1.02
CY2018Q1	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	-12.85
CY2019Q4	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Basic Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare	-0.25
CY2019Q3	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Basic Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare	0.28
CY2019Q2	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Basic Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare	0
CY2019Q1	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Basic Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare	0.22
CY2018Q4	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Basic Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare	0
CY2018Q3	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Basic Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare	-5.44
CY2018Q2	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Basic Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare	0
CY2018Q1	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Basic Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare	0.03
CY2019Q4	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	19.12
CY2019Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	15.29
CY2019Q2	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	6.44
CY2019Q1	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	89.64
CY2018Q4	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-43.24
CY2018Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	12.83
CY2018Q2	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.02
CY2018Q1	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-12.82
CY2019Q4	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	19.37
CY2019Q3	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	15.01
CY2019Q2	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	6.44
CY2019Q1	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	89.42
CY2018Q4	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	-43.24
CY2018Q3	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	18.27
CY2018Q2	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	1.02
CY2018Q1	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	-12.85
CY2019Q4	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Diluted Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare	-0.25
CY2019Q3	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Diluted Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare	0.28
CY2019Q2	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Diluted Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare	0
CY2019Q1	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Diluted Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare	0.22
CY2018Q4	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Diluted Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare	0
CY2018Q3	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Diluted Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare	-5.44
CY2018Q2	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Diluted Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare	0
CY2018Q1	us-gaap	Income Loss From Discontinued Operations Net Of Tax Per Diluted Share IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare	0.03
CY2019Q4	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	19.12
CY2019Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	15.29
CY2019Q2	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	6.44
CY2019Q1	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	89.64
CY2018Q4	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-43.24
CY2018Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	12.83
CY2018Q2	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.02
CY2018Q1	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-12.82
CY2019Q4	us-gaap	Summary Of Investments Other Than Investments In Related Parties Cost SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost	2599800000	USD
CY2019Q4	us-gaap	Summary Of Investments Other Than Investments In Related Parties Carrying Amount SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount	2947200000	USD
CY2019Q4	us-gaap	Summary Of Investments Other Than Investments In Related Parties Fair Value SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue	2947200000	USD
CY2019	wtm	Amortizationof Restricted Sharesand Option Awards AmortizationofRestrictedSharesandOptionAwards	10500000	USD
CY2018	wtm	Amortizationof Restricted Sharesand Option Awards AmortizationofRestrictedSharesandOptionAwards	13000000.0	USD
CY2017	wtm	Amortizationof Restricted Sharesand Option Awards AmortizationofRestrictedSharesandOptionAwards	14800000	USD
CY2019Q4	us-gaap	Due To Affiliate Current And Noncurrent DueToAffiliateCurrentAndNoncurrent	6600000	USD
CY2018Q4	us-gaap	Due To Affiliate Current And Noncurrent DueToAffiliateCurrentAndNoncurrent	4000000.0	USD
CY2017Q4	us-gaap	Due To Affiliate Current And Noncurrent DueToAffiliateCurrentAndNoncurrent	11600000	USD

Files In Submission

Name	View Source	Status
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0000776867-20-000009.txt	Edgar Link	pending
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wtm10-k2019_htm.xml	Edgar Link	completed

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$WTM
WHITE MOUNTAINS INSURANCE GROUP LTD Stock $wtm

2019 Form 10-K Financial Statement

#000077686720000009 Filed on March 02, 2020

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$WTM WHITE MOUNTAINS INSURANCE GROUP LTD Stock $wtm

2019 Form 10-K Financial Statement

#000077686720000009 Filed on March 02, 2020

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$WTM
WHITE MOUNTAINS INSURANCE GROUP LTD Stock $wtm