$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

$fibk Overview Financials Valuation SEC Filings Company Website

2023 Q1 Form 10-K Financial Statement

#000086041323000045 Filed on February 24, 2023

Income Statement

Concept	2023 Q1	2022 Q4	2022
Revenue	$315.9M		$1.022B
YoY Change	71.68%		102.08%
Cost Of Revenue
YoY Change
Gross Profit
YoY Change
Gross Profit Margin
Selling, General & Admin
YoY Change
% of Gross Profit
Research & Development
YoY Change
% of Gross Profit
Depreciation & Amortization	$13.70M	$14.10M	$55.50M
YoY Change	3.01%	28.18%	25.0%
% of Gross Profit
Operating Expenses
YoY Change
Operating Profit
YoY Change
Interest Expense	$77.00M		$78.90M
YoY Change	1183.33%		356.07%
% of Operating Profit
Other Income/Expense, Net
YoY Change
Pretax Income	$74.30M	$110.0M	$257.1M
YoY Change	-279.9%	65.41%	3.75%
Income Tax	$18.00M	$24.20M	$54.90M
% Of Pretax Income	24.23%	22.0%	21.35%
Net Earnings	$56.30M	$85.80M	$202.2M
YoY Change	-268.56%	67.91%	5.26%
Net Earnings / Revenue	17.82%		19.79%
Basic Earnings Per Share	$0.54		$1.96
Diluted Earnings Per Share	$0.54	$820.7K	$1.957M
COMMON SHARES
Basic Shares Outstanding	104.4M shares	104.4M shares	103.3M shares
Diluted Shares Outstanding	103.8M shares		103.3M shares

Balance Sheet

Concept	2023 Q1	2022 Q4	2022
SHORT-TERM ASSETS
Cash & Short-Term Investments		$871.7M
YoY Change		-90.15%
Cash & Equivalents	$1.081B	$870.5M
Short-Term Investments		$1.200M
Other Short-Term Assets
YoY Change
Inventory
Prepaid Expenses
Receivables
Other Receivables
Total Short-Term Assets
YoY Change
LONG-TERM ASSETS
Property, Plant & Equipment	$443.4M	$444.7M	$444.7M
YoY Change	-0.23%	48.43%	48.23%
Goodwill	$1.101B	$1.101B
YoY Change	-3.23%	77.11%
Intangibles
YoY Change
Long-Term Investments
YoY Change
Other Assets	$173.2M	$210.5M	$210.5M
YoY Change	26.06%
Total Long-Term Assets
YoY Change
TOTAL ASSETS
Total Short-Term Assets
Total Long-Term Assets
Total Assets	$31.64B	$32.29B	$32.29B
YoY Change	-4.6%	64.13%	64.13%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable	$387.9M	$460.4M	$460.4M
YoY Change	6365.0%	11410.0%	11410.0%
Accrued Expenses
YoY Change
Deferred Revenue
YoY Change
Short-Term Debt	$3.681B	$3.380B	$3.380B
YoY Change	243.68%	221.59%	221.59%
Long-Term Debt Due
YoY Change
Total Short-Term Liabilities	$4.069B	$460.4M	$460.4M
YoY Change	1429.01%	211.08%	211.08%
LONG-TERM LIABILITIES
Long-Term Debt	$283.9M	$283.9M	$283.9M
YoY Change	0.14%	42.66%	42.66%
Other Long-Term Liabilities	$17.80M	$476.6M	$476.6M
YoY Change	187.1%	11815.0%	11815.0%
Total Long-Term Liabilities	$301.7M	$760.5M	$760.5M
YoY Change	4.14%	274.63%	274.63%
TOTAL LIABILITIES
Total Short-Term Liabilities	$4.069B	$460.4M	$460.4M
Total Long-Term Liabilities	$301.7M	$760.5M	$760.5M
Total Liabilities	$28.48B	$29.21B	$29.21B
YoY Change	-4.18%	65.19%	65.19%
SHAREHOLDERS EQUITY
Retained Earnings	$1.081B	$1.073B
YoY Change	10.9%	1.91%
Common Stock		$0.00
YoY Change
Preferred Stock
YoY Change
Treasury Stock (at cost)
YoY Change
Treasury Stock Shares
Shareholders Equity	$3.160B	$3.074B	$3.074B
YoY Change
Total Liabilities & Shareholders Equity	$31.64B	$32.29B	$32.29B
YoY Change	-4.6%	64.13%	64.13%

Cashflow Statement

Concept	2023 Q1	2022 Q4	2022
OPERATING ACTIVITIES
Net Income	$56.30M	$85.80M	$202.2M
YoY Change	-268.56%	67.91%	5.26%
Depreciation, Depletion And Amortization	$13.70M	$14.10M	$55.50M
YoY Change	3.01%	28.18%	25.0%
Cash From Operating Activities	$57.70M	$95.80M	$534.4M
YoY Change	-11.09%	24.58%	89.3%
INVESTING ACTIVITIES
Capital Expenditures	$7.000M	-$16.70M	$10.50M
YoY Change	-173.68%	247.92%	-201.94%
Acquisitions
YoY Change
Other Investing Activities	$875.3M	-$652.7M	-$938.5M
YoY Change	-42.87%	170.49%	-55.52%
Cash From Investing Activities	$868.3M	-$636.0M	-$949.0M
YoY Change	-42.97%	158.43%	-55.24%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net
YoY Change
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	-$715.8M	818.8M	-$1.060B
YoY Change	492.06%	191.91%	-155.61%
NET CHANGE
Cash From Operating Activities	$57.70M	95.80M	$534.4M
Cash From Investing Activities	$868.3M	-636.0M	-$949.0M
Cash From Financing Activities	-$715.8M	818.8M	-$1.060B
Net Change In Cash	$210.2M	278.6M	-$1.474B
YoY Change	-85.67%	150.31%	-2268.09%
FREE CASH FLOW
Cash From Operating Activities	$57.70M	$95.80M	$534.4M
Capital Expenditures	$7.000M	-$16.70M	$10.50M
Free Cash Flow	$50.70M	$112.5M	$523.9M
YoY Change	-31.85%	37.7%	79.05%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2022	dei	Entity Central Index Key EntityCentralIndexKey	0000860413
CY2022	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2022	dei	Document Period End Date DocumentPeriodEndDate	2022-12-31
CY2022	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2022
CY2022	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
CY2022	dei	Amendment Flag AmendmentFlag	false
CY2022	dei	Document Type DocumentType	10-K
CY2022	dei	Document Annual Report DocumentAnnualReport	true
CY2022	dei	Document Transition Report DocumentTransitionReport	false
CY2022	dei	Entity File Number EntityFileNumber	001-34653
CY2022	dei	Entity Registrant Name EntityRegistrantName	FIRST INTERSTATE BANCSYSTEM, INC.
CY2022	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	MT
CY2022	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	81-0331430
CY2022	dei	Entity Address Address Line1 EntityAddressAddressLine1	401 North 31st Street
CY2022	dei	Entity Address City Or Town EntityAddressCityOrTown	Billings,
CY2022	dei	Entity Address State Or Province EntityAddressStateOrProvince	MT
CY2022	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	59101
CY2022	dei	City Area Code CityAreaCode	406
CY2022	dei	Local Phone Number LocalPhoneNumber	255-5311
CY2022	dei	Security12b Title Security12bTitle	Class A common stock, no par value
CY2022	dei	Trading Symbol TradingSymbol	FIBK
CY2022	dei	Security Exchange Name SecurityExchangeName	NASDAQ
CY2022	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	Yes
CY2022	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
CY2022	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2022	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
CY2022	dei	Entity Filer Category EntityFilerCategory	Large Accelerated Filer
CY2022	dei	Entity Small Business EntitySmallBusiness	false
CY2022	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
CY2022	dei	Icfr Auditor Attestation Flag IcfrAuditorAttestationFlag	true
CY2022	dei	Entity Shell Company EntityShellCompany	false
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CY2022	us-gaap	Interest Expense Other Short Term Borrowings InterestExpenseOtherShortTermBorrowings	15300000	usd
CY2021	us-gaap	Interest Expense Other Short Term Borrowings InterestExpenseOtherShortTermBorrowings	0	usd
CY2020	us-gaap	Interest Expense Other Short Term Borrowings InterestExpenseOtherShortTermBorrowings	0	usd
CY2022	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	6000000.0	usd
CY2021	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	6000000.0	usd
CY2020	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	4600000	usd
CY2022	us-gaap	Interest Expense Junior Subordinated Debentures InterestExpenseJuniorSubordinatedDebentures	6800000	usd
CY2021	us-gaap	Interest Expense Junior Subordinated Debentures InterestExpenseJuniorSubordinatedDebentures	2800000	usd
CY2020	us-gaap	Interest Expense Junior Subordinated Debentures InterestExpenseJuniorSubordinatedDebentures	3000000.0	usd
CY2022	us-gaap	Interest Expense InterestExpense	78900000	usd
CY2021	us-gaap	Interest Expense InterestExpense	17300000	usd
CY2020	us-gaap	Interest Expense InterestExpense	26600000	usd
CY2022	us-gaap	Interest Income Expense Net InterestIncomeExpenseNet	942600000	usd
CY2021	us-gaap	Interest Income Expense Net InterestIncomeExpenseNet	489200000	usd
CY2020	us-gaap	Interest Income Expense Net InterestIncomeExpenseNet	497800000	usd
CY2022	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	82700000	usd
CY2021	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	-14600000	usd
CY2020	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	56900000	usd
CY2022	us-gaap	Interest Income Expense After Provision For Loan Loss InterestIncomeExpenseAfterProvisionForLoanLoss	859900000	usd
CY2021	us-gaap	Interest Income Expense After Provision For Loan Loss InterestIncomeExpenseAfterProvisionForLoanLoss	503800000	usd
CY2020	us-gaap	Interest Income Expense After Provision For Loan Loss InterestIncomeExpenseAfterProvisionForLoanLoss	440900000	usd
CY2022	fibk	Payment Services Revenues PaymentServicesRevenues	74100000	usd
CY2021	fibk	Payment Services Revenues PaymentServicesRevenues	45100000	usd
CY2020	fibk	Payment Services Revenues PaymentServicesRevenues	41100000	usd
CY2022	us-gaap	Fees And Commissions Mortgage Banking And Servicing FeesAndCommissionsMortgageBankingAndServicing	18700000	usd
CY2021	us-gaap	Fees And Commissions Mortgage Banking And Servicing FeesAndCommissionsMortgageBankingAndServicing	40800000	usd
CY2020	us-gaap	Fees And Commissions Mortgage Banking And Servicing FeesAndCommissionsMortgageBankingAndServicing	47300000	usd
CY2022	fibk	Wealth Mangement Revenues WealthMangementRevenues	34300000	usd
CY2021	fibk	Wealth Mangement Revenues WealthMangementRevenues	26300000	usd
CY2020	fibk	Wealth Mangement Revenues WealthMangementRevenues	23800000	usd
CY2022	fibk	Service Charges On Deposit Accounts ServiceChargesOnDepositAccounts	24600000	usd
CY2021	fibk	Service Charges On Deposit Accounts ServiceChargesOnDepositAccounts	16500000	usd
CY2020	fibk	Service Charges On Deposit Accounts ServiceChargesOnDepositAccounts	17600000	usd
CY2022	fibk	Other Service Charges Commissions And Fees OtherServiceChargesCommissionsAndFees	15500000	usd
CY2021	fibk	Other Service Charges Commissions And Fees OtherServiceChargesCommissionsAndFees	7900000	usd
CY2020	fibk	Other Service Charges Commissions And Fees OtherServiceChargesCommissionsAndFees	12100000	usd
CY2022	us-gaap	Debt Securities Available For Sale Gain Loss DebtSecuritiesAvailableForSaleGainLoss	-24400000	usd
CY2021	us-gaap	Debt Securities Available For Sale Gain Loss DebtSecuritiesAvailableForSaleGainLoss	1100000	usd
CY2020	us-gaap	Debt Securities Available For Sale Gain Loss DebtSecuritiesAvailableForSaleGainLoss	300000	usd
CY2022	us-gaap	Noninterest Income Other Operating Income NoninterestIncomeOtherOperatingIncome	20400000	usd
CY2021	us-gaap	Noninterest Income Other Operating Income NoninterestIncomeOtherOperatingIncome	11800000	usd
CY2020	us-gaap	Noninterest Income Other Operating Income NoninterestIncomeOtherOperatingIncome	13700000	usd
CY2022	us-gaap	Noninterest Income NoninterestIncome	163200000	usd
CY2021	us-gaap	Noninterest Income NoninterestIncome	149500000	usd
CY2020	us-gaap	Noninterest Income NoninterestIncome	155900000	usd
CY2022	us-gaap	Labor And Related Expense LaborAndRelatedExpense	282100000	usd
CY2021	us-gaap	Labor And Related Expense LaborAndRelatedExpense	164900000	usd
CY2020	us-gaap	Labor And Related Expense LaborAndRelatedExpense	173700000	usd
CY2022	us-gaap	Other Labor Related Expenses OtherLaborRelatedExpenses	77500000	usd
CY2021	us-gaap	Other Labor Related Expenses OtherLaborRelatedExpenses	55800000	usd
CY2020	us-gaap	Other Labor Related Expenses OtherLaborRelatedExpenses	49400000	usd
CY2022	us-gaap	Information Technology And Data Processing InformationTechnologyAndDataProcessing	54300000	usd
CY2021	us-gaap	Information Technology And Data Processing InformationTechnologyAndDataProcessing	32800000	usd
CY2020	us-gaap	Information Technology And Data Processing InformationTechnologyAndDataProcessing	32800000	usd
CY2022	us-gaap	Occupancy Net OccupancyNet	44000000.0	usd
CY2021	us-gaap	Occupancy Net OccupancyNet	28700000	usd
CY2020	us-gaap	Occupancy Net OccupancyNet	28500000	usd
CY2022	us-gaap	Equipment Expense EquipmentExpense	23400000	usd
CY2021	us-gaap	Equipment Expense EquipmentExpense	17600000	usd
CY2020	us-gaap	Equipment Expense EquipmentExpense	15500000	usd
CY2022	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	2300000	usd
CY2021	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	-200000	usd
CY2021	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	-200000	usd
CY2020	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	-500000	usd
CY2020	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	-500000	usd
CY2022	us-gaap	Professional Fees ProfessionalFees	19100000	usd
CY2021	us-gaap	Professional Fees ProfessionalFees	12100000	usd
CY2020	us-gaap	Professional Fees ProfessionalFees	10900000	usd
CY2022	us-gaap	Federal Deposit Insurance Corporation Premium Expense FederalDepositInsuranceCorporationPremiumExpense	14000000.0	usd
CY2021	us-gaap	Federal Deposit Insurance Corporation Premium Expense FederalDepositInsuranceCorporationPremiumExpense	6600000	usd
CY2020	us-gaap	Federal Deposit Insurance Corporation Premium Expense FederalDepositInsuranceCorporationPremiumExpense	5900000	usd
CY2022	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	15900000	usd
CY2021	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	9900000	usd
CY2020	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	10900000	usd
CY2022	us-gaap	Other Noninterest Expense OtherNoninterestExpense	114500000	usd
CY2021	us-gaap	Other Noninterest Expense OtherNoninterestExpense	65700000	usd
CY2020	us-gaap	Other Noninterest Expense OtherNoninterestExpense	60400000	usd
CY2022	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	118900000	usd
CY2021	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	11600000	usd
CY2020	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	0	usd
CY2022	us-gaap	Noninterest Expense NoninterestExpense	766000000.0	usd
CY2021	us-gaap	Noninterest Expense NoninterestExpense	405500000	usd
CY2020	us-gaap	Noninterest Expense NoninterestExpense	387500000	usd
CY2022	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	257100000	usd
CY2021	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	247800000	usd
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	209300000	usd
CY2022	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	54900000	usd
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	55700000	usd
CY2020	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	48100000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	202200000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	usd
CY2020	us-gaap	Net Income Loss NetIncomeLoss	161200000	usd
CY2022	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.96
CY2021	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	3.12
CY2020	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.53
CY2022	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.96
CY2021	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	3.11
CY2020	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.53
CY2022	us-gaap	Net Income Loss NetIncomeLoss	202200000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	usd
CY2020	us-gaap	Net Income Loss NetIncomeLoss	161200000	usd
CY2022	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	-613100000	usd
CY2021	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	-113700000	usd
CY2020	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	61800000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	-24400000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	1100000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	300000	usd
CY2022	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	200000	usd
CY2021	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	0	usd
CY2020	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	0	usd
CY2022	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Before Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossBeforeTax	26100000	usd
CY2021	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Before Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossBeforeTax	-20200000	usd
CY2020	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Before Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossBeforeTax	0	usd
CY2022	us-gaap	Unrealized Gain Loss On Derivatives UnrealizedGainLossOnDerivatives	-6700000	usd
CY2021	us-gaap	Unrealized Gain Loss On Derivatives UnrealizedGainLossOnDerivatives	4200000	usd
CY2020	us-gaap	Unrealized Gain Loss On Derivatives UnrealizedGainLossOnDerivatives	200000	usd
CY2022	us-gaap	Defined Benefit Plan Actuarial Gain Loss DefinedBenefitPlanActuarialGainLoss	0	usd
CY2021	us-gaap	Defined Benefit Plan Actuarial Gain Loss DefinedBenefitPlanActuarialGainLoss	0	usd
CY2020	us-gaap	Defined Benefit Plan Actuarial Gain Loss DefinedBenefitPlanActuarialGainLoss	-500000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-621300000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-90400000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	61200000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-155200000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-22800000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	15600000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-466100000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-67600000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	45600000	usd
CY2022	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-263900000	usd
CY2021	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	124500000	usd
CY2020	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	206800000	usd
CY2019Q4	us-gaap	Stockholders Equity StockholdersEquity	2013900000	usd
CY2020	us-gaap	Net Income Loss NetIncomeLoss	161200000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	45600000	usd
CY2020	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	3578743	shares
CY2020	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	116800000	usd
CY2020	us-gaap	Stock Issued During Period Shares New Issues StockIssuedDuringPeriodSharesNewIssues	19491	shares
CY2020	us-gaap	Stock Issued During Period Value New Issues StockIssuedDuringPeriodValueNewIssues	0	usd
CY2020	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	332085	shares
CY2020Q4	us-gaap	Common Stock Value CommonStockValue	0	usd
CY2020	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Nonvested Options Forfeited Number Of Shares SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedNumberOfShares	34912	shares
CY2020	us-gaap	Stock Issued During Period Value Restricted Stock Award Forfeitures StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0	usd
CY2020	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	111539	shares
CY2020	fibk	Shares Tendered In Payment Of Option Price And Income Tax Withholding Amounts Shares SharesTenderedInPaymentOfOptionPriceAndIncomeTaxWithholdingAmountsShares	26124	shares
CY2020	us-gaap	Stock Issued During Period Value Stock Options Exercised Net Of Tax Benefit Expense StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	1100000	usd
CY2020	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	7500000	usd
CY2020	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	2.00
CY2020	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Stock Options Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	-128600000	usd
CY2020Q4	us-gaap	Stockholders Equity StockholdersEquity	1959800000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-67600000	usd
CY2021	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	128171	shares
CY2021	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	5400000	usd
CY2021	us-gaap	Stock Issued During Period Shares New Issues StockIssuedDuringPeriodSharesNewIssues	19081	shares
CY2021	us-gaap	Stock Issued During Period Value New Issues StockIssuedDuringPeriodValueNewIssues	0	usd
CY2021	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	241307	shares
CY2021Q4	us-gaap	Common Stock Value CommonStockValue	0	usd
CY2021	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Nonvested Options Forfeited Number Of Shares SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedNumberOfShares	73044	shares
CY2021	us-gaap	Stock Issued During Period Value Restricted Stock Award Forfeitures StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0	usd
CY2021	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	45484	shares
CY2021	fibk	Shares Tendered In Payment Of Option Price And Income Tax Withholding Amounts Shares SharesTenderedInPaymentOfOptionPriceAndIncomeTaxWithholdingAmountsShares	6982	shares
CY2021	us-gaap	Stock Issued During Period Value Stock Options Exercised Net Of Tax Benefit Expense StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	400000	usd
CY2021	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	8900000	usd
CY2021	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.64
CY2021	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Stock Options Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	-101600000	usd
CY2021Q4	us-gaap	Stockholders Equity StockholdersEquity	1986600000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	202200000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-466100000	usd
CY2022	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	5040896	shares
CY2022	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	199000000.0	usd
CY2022	us-gaap	Stock Issued During Period Shares New Issues StockIssuedDuringPeriodSharesNewIssues	46913370	shares
CY2022	us-gaap	Stock Issued During Period Value New Issues StockIssuedDuringPeriodValueNewIssues	1722500000	usd
CY2022	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	458177	shares
CY2022Q4	us-gaap	Common Stock Value CommonStockValue	0	usd
CY2022	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Nonvested Options Forfeited Number Of Shares SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedNumberOfShares	106891	shares
CY2022	us-gaap	Stock Issued During Period Value Restricted Stock Award Forfeitures StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0	usd
CY2022	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	17807	shares
CY2022	fibk	Shares Tendered In Payment Of Option Price And Income Tax Withholding Amounts Shares SharesTenderedInPaymentOfOptionPriceAndIncomeTaxWithholdingAmountsShares	4877	shares
CY2022	us-gaap	Stock Issued During Period Value Stock Options Exercised Net Of Tax Benefit Expense StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	100000	usd
CY2022	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	9600000	usd
CY2022	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.70
CY2022	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Stock Options Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	-182100000	usd
CY2022Q4	us-gaap	Stockholders Equity StockholdersEquity	3073800000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	202200000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	usd
CY2020	us-gaap	Net Income Loss NetIncomeLoss	161200000	usd
CY2022	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	82700000	usd
CY2021	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	-14600000	usd
CY2020	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	56900000	usd
CY2022	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	2000000.0	usd
CY2021	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	1800000	usd
CY2020	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	-300000	usd
CY2022	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	55500000	usd
CY2021	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	44400000	usd
CY2020	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	45100000	usd
CY2022	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	-17000000.0	usd
CY2021	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	-38800000	usd
CY2020	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	-15900000	usd
CY2022	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	-24400000	usd
CY2021	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	1100000	usd
CY2020	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	300000	usd
CY2022	us-gaap	Gain Loss On Sale Of Loans And Leases GainLossOnSaleOfLoansAndLeases	9400000	usd
CY2021	us-gaap	Gain Loss On Sale Of Loans And Leases GainLossOnSaleOfLoansAndLeases	26200000	usd
CY2020	us-gaap	Gain Loss On Sale Of Loans And Leases GainLossOnSaleOfLoansAndLeases	49300000	usd
CY2022	us-gaap	Gain Loss On Sale Of Other Investments GainLossOnSaleOfOtherInvestments	0	usd
CY2021	us-gaap	Gain Loss On Sale Of Other Investments GainLossOnSaleOfOtherInvestments	0	usd
CY2020	us-gaap	Gain Loss On Sale Of Other Investments GainLossOnSaleOfOtherInvestments	1000000.0	usd
CY2022	us-gaap	Gains Losses On Sales Of Other Real Estate GainsLossesOnSalesOfOtherRealEstate	700000	usd
CY2021	us-gaap	Gains Losses On Sales Of Other Real Estate GainsLossesOnSalesOfOtherRealEstate	300000	usd
CY2020	us-gaap	Gains Losses On Sales Of Other Real Estate GainsLossesOnSalesOfOtherRealEstate	900000	usd
CY2022	fibk	Impairmentof Real Estateand Other Assets Pending Disposal ImpairmentofRealEstateandOtherAssetsPendingDisposal	2800000	usd
CY2021	fibk	Impairmentof Real Estateand Other Assets Pending Disposal ImpairmentofRealEstateandOtherAssetsPendingDisposal	0	usd
CY2020	fibk	Impairmentof Real Estateand Other Assets Pending Disposal ImpairmentofRealEstateandOtherAssetsPendingDisposal	100000	usd
CY2022	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	1400000	usd
CY2021	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	0	usd
CY2020	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	0	usd
CY2022	us-gaap	Mortgage Servicing Rights Msr Impairment Recovery MortgageServicingRightsMSRImpairmentRecovery	-3400000	usd
CY2021	us-gaap	Mortgage Servicing Rights Msr Impairment Recovery MortgageServicingRightsMSRImpairmentRecovery	-6900000	usd
CY2020	us-gaap	Mortgage Servicing Rights Msr Impairment Recovery MortgageServicingRightsMSRImpairmentRecovery	9900000	usd
CY2022	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-4700000	usd
CY2021	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	5000000.0	usd
CY2020	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-6600000	usd
CY2022	us-gaap	Life Insurance Corporate Or Bank Owned Change In Value LifeInsuranceCorporateOrBankOwnedChangeInValue	11100000	usd
CY2021	us-gaap	Life Insurance Corporate Or Bank Owned Change In Value LifeInsuranceCorporateOrBankOwnedChangeInValue	6100000	usd
CY2020	us-gaap	Life Insurance Corporate Or Bank Owned Change In Value LifeInsuranceCorporateOrBankOwnedChangeInValue	7600000	usd
CY2022	us-gaap	Share Based Compensation ShareBasedCompensation	9600000	usd
CY2021	us-gaap	Share Based Compensation ShareBasedCompensation	8900000	usd
CY2020	us-gaap	Share Based Compensation ShareBasedCompensation	7500000	usd
CY2022	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	429500000	usd
CY2021	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	817400000	usd
CY2020	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	1404200000	usd
CY2022	us-gaap	Proceeds From Sale Of Mortgage Loans Held For Sale ProceedsFromSaleOfMortgageLoansHeldForSale	461600000	usd
CY2021	us-gaap	Proceeds From Sale Of Mortgage Loans Held For Sale ProceedsFromSaleOfMortgageLoansHeldForSale	883900000	usd
CY2020	us-gaap	Proceeds From Sale Of Mortgage Loans Held For Sale ProceedsFromSaleOfMortgageLoansHeldForSale	1468400000	usd
CY2022	us-gaap	Increase Decrease In Accrued Interest Receivable Net IncreaseDecreaseInAccruedInterestReceivableNet	37800000	usd
CY2021	us-gaap	Increase Decrease In Accrued Interest Receivable Net IncreaseDecreaseInAccruedInterestReceivableNet	-3700000	usd
CY2020	us-gaap	Increase Decrease In Accrued Interest Receivable Net IncreaseDecreaseInAccruedInterestReceivableNet	4400000	usd
CY2022	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	12900000	usd
CY2021	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	16400000	usd
CY2020	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	27700000	usd
CY2022	us-gaap	Increase Decrease In Interest Payable Net IncreaseDecreaseInInterestPayableNet	8400000	usd
CY2021	us-gaap	Increase Decrease In Interest Payable Net IncreaseDecreaseInInterestPayableNet	-2000000.0	usd
CY2020	us-gaap	Increase Decrease In Interest Payable Net IncreaseDecreaseInInterestPayableNet	-6300000	usd
CY2022	us-gaap	Increase Decrease In Accounts Payable And Accrued Liabilities IncreaseDecreaseInAccountsPayableAndAccruedLiabilities	183100000	usd
CY2021	us-gaap	Increase Decrease In Accounts Payable And Accrued Liabilities IncreaseDecreaseInAccountsPayableAndAccruedLiabilities	-1700000	usd
CY2020	us-gaap	Increase Decrease In Accounts Payable And Accrued Liabilities IncreaseDecreaseInAccountsPayableAndAccruedLiabilities	11300000	usd
CY2022	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	534400000	usd
CY2021	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	282300000	usd
CY2020	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	268300000	usd
CY2022	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	858500000	usd
CY2021	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	1238000000	usd
CY2020	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	0	usd
CY2022	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	3309400000	usd
CY2021	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	2717800000	usd
CY2020	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	2444100000	usd
CY2022	us-gaap	Proceeds From Sale And Maturity Of Held To Maturity Securities ProceedsFromSaleAndMaturityOfHeldToMaturitySecurities	370500000	usd
CY2021	us-gaap	Proceeds From Sale And Maturity Of Held To Maturity Securities ProceedsFromSaleAndMaturityOfHeldToMaturitySecurities	257600000	usd
CY2020	us-gaap	Proceeds From Sale And Maturity Of Held To Maturity Securities ProceedsFromSaleAndMaturityOfHeldToMaturitySecurities	40400000	usd
CY2022	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	1926500000	usd
CY2021	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	1118000000	usd
CY2020	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	1441400000	usd
CY2022	fibk	Purchases Of Federal Home Loan Bank And Federal Reserve Bank Stock PurchasesOfFederalHomeLoanBankAndFederalReserveBankStock	241400000	usd
CY2021	fibk	Purchases Of Federal Home Loan Bank And Federal Reserve Bank Stock PurchasesOfFederalHomeLoanBankAndFederalReserveBankStock	0	usd
CY2020	fibk	Purchases Of Federal Home Loan Bank And Federal Reserve Bank Stock PurchasesOfFederalHomeLoanBankAndFederalReserveBankStock	0	usd
CY2022	fibk	Proceeds From Federal Home Loan Bank And Federal Reserve Bank Stock ProceedsFromFederalHomeLoanBankAndFederalReserveBankStock	112200000	usd
CY2021	fibk	Proceeds From Federal Home Loan Bank And Federal Reserve Bank Stock ProceedsFromFederalHomeLoanBankAndFederalReserveBankStock	0	usd
CY2020	fibk	Proceeds From Federal Home Loan Bank And Federal Reserve Bank Stock ProceedsFromFederalHomeLoanBankAndFederalReserveBankStock	0	usd
CY2022	us-gaap	Proceeds From Life Insurance Policies ProceedsFromLifeInsurancePolicies	1300000	usd
CY2021	us-gaap	Proceeds From Life Insurance Policies ProceedsFromLifeInsurancePolicies	1000000.0	usd
CY2020	us-gaap	Proceeds From Life Insurance Policies ProceedsFromLifeInsurancePolicies	5000000.0	usd
CY2022	us-gaap	Payments For Proceeds From Other Loans And Leases PaymentsForProceedsFromOtherLoansAndLeases	951400000	usd
CY2021	us-gaap	Payments For Proceeds From Other Loans And Leases PaymentsForProceedsFromOtherLoansAndLeases	-467800000	usd
CY2020	us-gaap	Payments For Proceeds From Other Loans And Leases PaymentsForProceedsFromOtherLoansAndLeases	894600000	usd
CY2022	us-gaap	Proceeds From Sale Of Wholly Owned Real Estate And Real Estate Acquired In Settlement Of Loans ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans	3400000	usd
CY2021	us-gaap	Proceeds From Sale Of Wholly Owned Real Estate And Real Estate Acquired In Settlement Of Loans ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans	1700000	usd
CY2020	us-gaap	Proceeds From Sale Of Wholly Owned Real Estate And Real Estate Acquired In Settlement Of Loans ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans	10100000	usd
CY2022	fibk	Proceeds From The Sale Of Health Savings Accounts ProceedsFromTheSaleOfHealthSavingsAccounts	1400000	usd
CY2021	fibk	Proceeds From The Sale Of Health Savings Accounts ProceedsFromTheSaleOfHealthSavingsAccounts	0	usd
CY2020	fibk	Proceeds From The Sale Of Health Savings Accounts ProceedsFromTheSaleOfHealthSavingsAccounts	0	usd
CY2022	us-gaap	Proceeds From Sale Of Other Investments ProceedsFromSaleOfOtherInvestments	0	usd
CY2021	us-gaap	Proceeds From Sale Of Other Investments ProceedsFromSaleOfOtherInvestments	0	usd
CY2020	us-gaap	Proceeds From Sale Of Other Investments ProceedsFromSaleOfOtherInvestments	2200000	usd
CY2022	us-gaap	Cash Acquired From Acquisition CashAcquiredFromAcquisition	2006900000	usd
CY2021	us-gaap	Cash Acquired From Acquisition CashAcquiredFromAcquisition	0	usd
CY2020	us-gaap	Cash Acquired From Acquisition CashAcquiredFromAcquisition	0	usd
CY2022	us-gaap	Payments For Proceeds From Productive Assets PaymentsForProceedsFromProductiveAssets	10500000	usd
CY2021	us-gaap	Payments For Proceeds From Productive Assets PaymentsForProceedsFromProductiveAssets	10300000	usd
CY2020	us-gaap	Payments For Proceeds From Productive Assets PaymentsForProceedsFromProductiveAssets	30200000	usd
CY2022	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-949000000.0	usd
CY2021	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-2120000000	usd
CY2020	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-1869800000	usd
CY2022	us-gaap	Increase Decrease In Deposits IncreaseDecreaseInDeposits	-2884000000	usd
CY2021	us-gaap	Increase Decrease In Deposits IncreaseDecreaseInDeposits	2052600000	usd
CY2020	us-gaap	Increase Decrease In Deposits IncreaseDecreaseInDeposits	2553500000	usd
CY2022	us-gaap	Proceeds From Payments For In Securities Sold Under Agreements To Repurchase ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase	28900000	usd
CY2021	us-gaap	Proceeds From Payments For In Securities Sold Under Agreements To Repurchase ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase	-40300000	usd
CY2020	us-gaap	Proceeds From Payments For In Securities Sold Under Agreements To Repurchase ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase	393800000	usd
CY2022	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	2327000000	usd
CY2021	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	0	usd
CY2020	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	0	usd
CY2022	us-gaap	Repayments Of Other Long Term Debt RepaymentsOfOtherLongTermDebt	164100000	usd
CY2021	us-gaap	Repayments Of Other Long Term Debt RepaymentsOfOtherLongTermDebt	0	usd
CY2020	us-gaap	Repayments Of Other Long Term Debt RepaymentsOfOtherLongTermDebt	100000	usd
CY2022	us-gaap	Proceeds From Issuance Of Unsecured Debt ProceedsFromIssuanceOfUnsecuredDebt	14300000	usd
CY2021	us-gaap	Proceeds From Issuance Of Unsecured Debt ProceedsFromIssuanceOfUnsecuredDebt	0	usd
CY2020	us-gaap	Proceeds From Issuance Of Unsecured Debt ProceedsFromIssuanceOfUnsecuredDebt	98600000	usd
CY2022	us-gaap	Payments Of Stock Issuance Costs PaymentsOfStockIssuanceCosts	800000	usd
CY2021	us-gaap	Payments Of Stock Issuance Costs PaymentsOfStockIssuanceCosts	0	usd
CY2020	us-gaap	Payments Of Stock Issuance Costs PaymentsOfStockIssuanceCosts	0	usd
CY2022	us-gaap	Proceeds From Issuance Of Common Stock ProceedsFromIssuanceOfCommonStock	100000	usd
CY2021	us-gaap	Proceeds From Issuance Of Common Stock ProceedsFromIssuanceOfCommonStock	400000	usd
CY2020	us-gaap	Proceeds From Issuance Of Common Stock ProceedsFromIssuanceOfCommonStock	1100000	usd
CY2022	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	199000000.0	usd
CY2021	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	5400000	usd
CY2020	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	116800000	usd
CY2022	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	182100000	usd
CY2021	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	101600000	usd
CY2020	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	128600000	usd
CY2022	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-1059700000	usd
CY2021	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	1905700000	usd
CY2020	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	2801500000	usd
CY2022	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	-1474300000	usd
CY2021	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	68000000.0	usd
CY2020	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	1200000000	usd
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	2344800000	usd
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	2276800000	usd
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	1076800000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	870500000	usd
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	2344800000	usd
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	2276800000	usd
CY2022	us-gaap	Income Taxes Paid IncomeTaxesPaid	57800000	usd
CY2021	us-gaap	Income Taxes Paid IncomeTaxesPaid	56800000	usd
CY2020	us-gaap	Income Taxes Paid IncomeTaxesPaid	54400000	usd
CY2022	us-gaap	Interest Paid Net InterestPaidNet	32000000.0	usd
CY2021	us-gaap	Interest Paid Net InterestPaidNet	19300000	usd
CY2020	us-gaap	Interest Paid Net InterestPaidNet	32900000	usd
CY2022	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	22900000	usd
CY2021	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	5900000	usd
CY2020	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	3600000	usd
CY2022	us-gaap	Proceeds From Sale Of Held To Maturity Securities ProceedsFromSaleOfHeldToMaturitySecurities	10900000	usd
CY2021	us-gaap	Proceeds From Sale Of Held To Maturity Securities ProceedsFromSaleOfHeldToMaturitySecurities	0	usd
CY2020	us-gaap	Proceeds From Sale Of Held To Maturity Securities ProceedsFromSaleOfHeldToMaturitySecurities	0	usd
CY2022	fibk	Non Cash Transfer Of Securities From Available For Sale To Held To Maturity NonCashTransferOfSecuritiesFromAvailableForSaleToHeldToMaturity	463600000	usd
CY2021	fibk	Non Cash Transfer Of Securities From Available For Sale To Held To Maturity NonCashTransferOfSecuritiesFromAvailableForSaleToHeldToMaturity	672200000	usd
CY2020	fibk	Non Cash Transfer Of Securities From Available For Sale To Held To Maturity NonCashTransferOfSecuritiesFromAvailableForSaleToHeldToMaturity	0	usd
CY2022	us-gaap	Transfer Of Loans Held For Sale To Portfolio Loans1 TransferOfLoansHeldForSaleToPortfolioLoans1	19800000	usd
CY2021	us-gaap	Transfer Of Loans Held For Sale To Portfolio Loans1 TransferOfLoansHeldForSaleToPortfolioLoans1	0	usd
CY2020	us-gaap	Transfer Of Loans Held For Sale To Portfolio Loans1 TransferOfLoansHeldForSaleToPortfolioLoans1	0	usd
CY2022	us-gaap	Transfer Of Portfolio Loans And Leases To Held For Sale1 TransferOfPortfolioLoansAndLeasesToHeldForSale1	12400000	usd
CY2021	us-gaap	Transfer Of Portfolio Loans And Leases To Held For Sale1 TransferOfPortfolioLoansAndLeasesToHeldForSale1	0	usd
CY2020	us-gaap	Transfer Of Portfolio Loans And Leases To Held For Sale1 TransferOfPortfolioLoansAndLeasesToHeldForSale1	0	usd
CY2022	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	400000	usd
CY2021	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	900000	usd
CY2020	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	3300000	usd
CY2022	us-gaap	Asset Acquisition Consideration Transferred Equity Interest Issued And Issuable AssetAcquisitionConsiderationTransferredEquityInterestIssuedAndIssuable	1723300000	usd
CY2021	us-gaap	Asset Acquisition Consideration Transferred Equity Interest Issued And Issuable AssetAcquisitionConsiderationTransferredEquityInterestIssuedAndIssuable	0	usd
CY2020	us-gaap	Asset Acquisition Consideration Transferred Equity Interest Issued And Issuable AssetAcquisitionConsiderationTransferredEquityInterestIssuedAndIssuable	0	usd
CY2022	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	2500000	usd
CY2021	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	3700000	usd
CY2020	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	11700000	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Investments Acquired1 NoncashOrPartNoncashAcquisitionInvestmentsAcquired1	2699000000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Investments Acquired1 NoncashOrPartNoncashAcquisitionInvestmentsAcquired1	0	usd
CY2020	us-gaap	Noncash Or Part Noncash Acquisition Investments Acquired1 NoncashOrPartNoncashAcquisitionInvestmentsAcquired1	0	usd
CY2022	us-gaap	Securities Purchased Under Agreements To Resell Increase SecuritiesPurchasedUnderAgreementsToResellIncrease	101100000	usd
CY2021	us-gaap	Securities Purchased Under Agreements To Resell Increase SecuritiesPurchasedUnderAgreementsToResellIncrease	0	usd
CY2020	us-gaap	Securities Purchased Under Agreements To Resell Increase SecuritiesPurchasedUnderAgreementsToResellIncrease	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Loans Held For Sale Acquired NoncashOrPartNoncashAcquisitionLoansHeldForSaleAcquired	217000000.0	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Loans Held For Sale Acquired NoncashOrPartNoncashAcquisitionLoansHeldForSaleAcquired	0	usd
CY2020	fibk	Noncash Or Part Noncash Acquisition Loans Held For Sale Acquired NoncashOrPartNoncashAcquisitionLoansHeldForSaleAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Loans Acquired NoncashOrPartNoncashAcquisitionLoansAcquired	7645500000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Loans Acquired NoncashOrPartNoncashAcquisitionLoansAcquired	0	usd
CY2020	fibk	Noncash Or Part Noncash Acquisition Loans Acquired NoncashOrPartNoncashAcquisitionLoansAcquired	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Fixed Assets Acquired1 NoncashOrPartNoncashAcquisitionFixedAssetsAcquired1	144700000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Fixed Assets Acquired1 NoncashOrPartNoncashAcquisitionFixedAssetsAcquired1	0	usd
CY2020	us-gaap	Noncash Or Part Noncash Acquisition Fixed Assets Acquired1 NoncashOrPartNoncashAcquisitionFixedAssetsAcquired1	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Goodwill NoncashOrPartNoncashAcquisitionGoodwill	479300000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Goodwill NoncashOrPartNoncashAcquisitionGoodwill	0	usd
CY2020	fibk	Noncash Or Part Noncash Acquisition Goodwill NoncashOrPartNoncashAcquisitionGoodwill	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Intangible Assets Acquired1 NoncashOrPartNoncashAcquisitionIntangibleAssetsAcquired1	72900000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Intangible Assets Acquired1 NoncashOrPartNoncashAcquisitionIntangibleAssetsAcquired1	0	usd
CY2020	us-gaap	Noncash Or Part Noncash Acquisition Intangible Assets Acquired1 NoncashOrPartNoncashAcquisitionIntangibleAssetsAcquired1	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Mortgage Servicing Asset Acquired NoncashOrPartNoncashAcquisitionMortgageServicingAssetAcquired	1300000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Mortgage Servicing Asset Acquired NoncashOrPartNoncashAcquisitionMortgageServicingAssetAcquired	0	usd
CY2020	fibk	Noncash Or Part Noncash Acquisition Mortgage Servicing Asset Acquired NoncashOrPartNoncashAcquisitionMortgageServicingAssetAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Company Owned Life Insurance Acquired NoncashOrPartNoncashAcquisitionCompanyOwnedLifeInsuranceAcquired	186600000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Company Owned Life Insurance Acquired NoncashOrPartNoncashAcquisitionCompanyOwnedLifeInsuranceAcquired	0	usd
CY2020	fibk	Noncash Or Part Noncash Acquisition Company Owned Life Insurance Acquired NoncashOrPartNoncashAcquisitionCompanyOwnedLifeInsuranceAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Deferred Tax Assets Acquired NoncashOrPartNoncashAcquisitionDeferredTaxAssetsAcquired	60200000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Deferred Tax Assets Acquired NoncashOrPartNoncashAcquisitionDeferredTaxAssetsAcquired	0	usd
CY2020	fibk	Noncash Or Part Noncash Acquisition Deferred Tax Assets Acquired NoncashOrPartNoncashAcquisitionDeferredTaxAssetsAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Other Real Estate Owned Acquired NoncashOrPartNoncashAcquisitionOtherRealEstateOwnedAcquired	15800000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Other Real Estate Owned Acquired NoncashOrPartNoncashAcquisitionOtherRealEstateOwnedAcquired	0	usd
CY2020	fibk	Noncash Or Part Noncash Acquisition Other Real Estate Owned Acquired NoncashOrPartNoncashAcquisitionOtherRealEstateOwnedAcquired	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Other Assets Acquired1 NoncashOrPartNoncashAcquisitionOtherAssetsAcquired1	200800000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Other Assets Acquired1 NoncashOrPartNoncashAcquisitionOtherAssetsAcquired1	0	usd
CY2020	us-gaap	Noncash Or Part Noncash Acquisition Other Assets Acquired1 NoncashOrPartNoncashAcquisitionOtherAssetsAcquired1	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Value Of Assets Acquired1 NoncashOrPartNoncashAcquisitionValueOfAssetsAcquired1	11824200000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Value Of Assets Acquired1 NoncashOrPartNoncashAcquisitionValueOfAssetsAcquired1	0	usd
CY2020	us-gaap	Noncash Or Part Noncash Acquisition Value Of Assets Acquired1 NoncashOrPartNoncashAcquisitionValueOfAssetsAcquired1	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Deposits Acquired NoncashOrPartNoncashAcquisitionDepositsAcquired	11688000000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Deposits Acquired NoncashOrPartNoncashAcquisitionDepositsAcquired	0	usd
CY2020	fibk	Noncash Or Part Noncash Acquisition Deposits Acquired NoncashOrPartNoncashAcquisitionDepositsAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Securities Sold Under Repurchase Agreements NoncashOrPartNoncashAcquisitionSecuritiesSoldUnderRepurchaseAgreements	74000000.0	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Securities Sold Under Repurchase Agreements NoncashOrPartNoncashAcquisitionSecuritiesSoldUnderRepurchaseAgreements	0	usd
CY2020	fibk	Noncash Or Part Noncash Acquisition Securities Sold Under Repurchase Agreements NoncashOrPartNoncashAcquisitionSecuritiesSoldUnderRepurchaseAgreements	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Payables Assumed1 NoncashOrPartNoncashAcquisitionPayablesAssumed1	110400000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Payables Assumed1 NoncashOrPartNoncashAcquisitionPayablesAssumed1	0	usd
CY2020	us-gaap	Noncash Or Part Noncash Acquisition Payables Assumed1 NoncashOrPartNoncashAcquisitionPayablesAssumed1	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Debt Assumed1 NoncashOrPartNoncashAcquisitionDebtAssumed1	159300000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Debt Assumed1 NoncashOrPartNoncashAcquisitionDebtAssumed1	0	usd
CY2020	us-gaap	Noncash Or Part Noncash Acquisition Debt Assumed1 NoncashOrPartNoncashAcquisitionDebtAssumed1	0	usd
CY2022	fibk	Noncashor Part Noncash Acquisition Trust Preferred Securities Assumed NoncashorPartNoncashAcquisitionTrustPreferredSecuritiesAssumed	76100000	usd
CY2021	fibk	Noncashor Part Noncash Acquisition Trust Preferred Securities Assumed NoncashorPartNoncashAcquisitionTrustPreferredSecuritiesAssumed	0	usd
CY2020	fibk	Noncashor Part Noncash Acquisition Trust Preferred Securities Assumed NoncashorPartNoncashAcquisitionTrustPreferredSecuritiesAssumed	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Value Of Liabilities Assumed1 NoncashOrPartNoncashAcquisitionValueOfLiabilitiesAssumed1	12107800000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Value Of Liabilities Assumed1 NoncashOrPartNoncashAcquisitionValueOfLiabilitiesAssumed1	0	usd
CY2020	us-gaap	Noncash Or Part Noncash Acquisition Value Of Liabilities Assumed1 NoncashOrPartNoncashAcquisitionValueOfLiabilitiesAssumed1	0	usd
CY2022Q4	fibk	Numberof Subsidiary Banks NumberofSubsidiaryBanks	1	subsidiary
CY2022	us-gaap	Use Of Estimates UseOfEstimates	<span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:700;line-height:118%">Use of Estimates.</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%"> The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America (“GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and income and expenses during the reporting period. Actual results could differ from those estimates. Material estimates that are particularly susceptible to change relate to the determination of the allowance for credit losses, the valuation of goodwill and intangible assets, and fair valuations of investment securities and other financial instruments.</span>
CY2022Q4	fibk	Federal Reserve Bank Unrestricted Cash Held Carrying Value FederalReserveBankUnrestrictedCashHeldCarryingValue	518900000	usd
CY2021Q4	fibk	Federal Reserve Bank Unrestricted Cash Held Carrying Value FederalReserveBankUnrestrictedCashHeldCarryingValue	2166100000	usd
CY2022Q4	us-gaap	Accrued Investment Income Receivable AccruedInvestmentIncomeReceivable	38900000	usd
CY2021Q4	us-gaap	Accrued Investment Income Receivable AccruedInvestmentIncomeReceivable	16600000	usd
CY2022	us-gaap	Financing Receivable Allowance For Credit Losses Policy For Uncollectible Amounts FinancingReceivableAllowanceForCreditLossesPolicyForUncollectibleAmounts	<div style="margin-bottom:4pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-style:italic;font-weight:700;line-height:118%">Allowance for Credit Losses - Loans held for investment</span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">The allowance for credit losses is a valuation account that is deducted from the loans’ amortized cost basis to present the net amount expected to be collected over the life of the loans. Loans are charged off against the allowance when management believes the uncollectibility of a loan balance is confirmed. When forecasting expected recoveries, the amounts should not exceed the aggregate of amounts that have previously been or are expected to be charged-off loans. The Company has elected to not forecast recoveries.</span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">Management estimates the allowance balance using relevant available information, from internal and external sources, relating to past events, current conditions, and reasonable and supportable forecasts. Historical credit loss experience provides the basis for the estimation of expected credit losses. Adjustments to historical loss information are made for differences in current loan-specific risk characteristics such as differences in underwriting standards, portfolio mix, delinquency level, or term as well as for changes in environmental and economic conditions, such as changes in unemployment rates, property values, or other relevant factors.</span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">The allowance for credit losses is measured on a collective (pool) basis when similar risk characteristics exist.</span></div><div style="margin-bottom:8pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">The Company applies Probability of Default (PD) and Loss Given Default (LGD) methodologies for all portfolio segments. The Company uses a Transition Matrix (TM) for PD components of the methodology and a historical average for the LGD components of methodology. The PD and LGD is applied to the current principal balance as of the reporting date. The TM determines the PD by tracking the historical movement of loans between loan risk tiers over a defined period of time. The Company currently has 17 portfolio segments for which we track monthly movement between either risk ratings or delinquency band.</span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">While the TM functions similarly across all portfolio segments, generally speaking, commercial portfolios use the Company’s risk rating scale and consumer portfolios use the delinquency band. Loans using risk ratings are scored utilizing the Company’s risk rating scale. The risk rating scale is 1-10, with 1 being the best rating, 6 being a pass but on watch, and 7-10 being various stages of criticized loans. Risk ratings 8 or greater and in a non-accrual status are considered in a defaulted state. Loans using delinquency band are measured using a 5-grade band, with 1 being current and 5 being 90 or more days past due. </span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">The LGD used as the basis for the estimate of credit losses is comprised of the Company’s historical loss experience from 2008 to the current period, based on a migration analysis of our historical loss experience, designed to account for credit deterioration. The model compares the most recent period losses to prior period defaults to calculate the LGD, which is averaged over the historical observations. </span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">Economic scenarios and forecasts along with current portfolio conditions and trends are monitored and accounted for through the Company’s qualitative framework. The Company utilizes a one-year forecast period with immediate reversion to historical loss rates. </span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">The Company segments the loan portfolio into pools based on the following risk characteristics: financial asset type, collateral type, loan characteristics, credit characteristics, outstanding loan balances, contractual terms and prepayment assumptions, vintage, industry of borrower and concentrations, and historical or expected credit loss.</span></div>
CY2022Q4	us-gaap	Deferred Compensation Plan Assets DeferredCompensationPlanAssets	18700000	usd
CY2021Q4	us-gaap	Deferred Compensation Plan Assets DeferredCompensationPlanAssets	21400000	usd
CY2022Q4	us-gaap	Deferred Compensation Liability Current And Noncurrent DeferredCompensationLiabilityCurrentAndNoncurrent	18700000	usd
CY2021Q4	us-gaap	Deferred Compensation Liability Current And Noncurrent DeferredCompensationLiabilityCurrentAndNoncurrent	21400000	usd
CY2022	us-gaap	Impairment Of Long Lived Assets To Be Disposed Of ImpairmentOfLongLivedAssetsToBeDisposedOf	0	usd
CY2021	us-gaap	Impairment Of Long Lived Assets To Be Disposed Of ImpairmentOfLongLivedAssetsToBeDisposedOf	0	usd
CY2020	us-gaap	Impairment Of Long Lived Assets To Be Disposed Of ImpairmentOfLongLivedAssetsToBeDisposedOf	0	usd
CY2022Q4	us-gaap	Federal Reserve Bank Stock FederalReserveBankStock	93600000	usd
CY2021Q4	us-gaap	Federal Reserve Bank Stock FederalReserveBankStock	42800000	usd
CY2022Q4	us-gaap	Federal Home Loan Bank Stock FederalHomeLoanBankStock	103800000	usd
CY2021Q4	us-gaap	Federal Home Loan Bank Stock FederalHomeLoanBankStock	10600000	usd
CY2022Q4	us-gaap	Marketable Securities MarketableSecurities	1200000	usd
CY2021Q4	us-gaap	Marketable Securities MarketableSecurities	400000	usd
CY2022	us-gaap	Derivatives Methods Of Accounting Hedging Derivatives DerivativesMethodsOfAccountingHedgingDerivatives	<div style="margin-bottom:8pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:700;line-height:118%">Derivatives and Hedging Activities. </span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">For asset and liability management purposes, the Company enters into interest rate swap contracts to hedge against changes in forecasted cash flows due to interest rate exposures. Interest rate swaps are contracts in which a series of interest payments are exchanged over a prescribed period. The notional amount upon which the interest payments are based is not exchanged. </span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">The Company formally assesses, both at the hedge's inception and on an ongoing basis, whether the derivatives that are used in hedging transactions are highly effective in offsetting changes in cash flows of hedged items. The gain or loss on cash flow hedging instruments is initially reported as a component of other comprehensive income and subsequently reclassified into earnings in the same period during which the transaction affects earnings. </span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">When it is determined that a derivative is not highly effective as a hedge or that it has ceased to be a highly effective hedge, the Company discontinues hedge accounting prospectively when (a) it is determined that the derivative is no longer effective in offsetting changes in the cash flows of a hedged item (including forecasted transactions); (b) the derivative expires or is sold, terminated, or exercised; (c) the derivative is de-designated as a hedge instrument, because it is unlikely that a forecasted transaction will occur; or (d) management determines that designation of the derivative as a hedge instrument is no longer appropriate.</span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">When hedge accounting is discontinued because it is probable that a forecasted transaction will not occur, the derivative will continue to be carried on the balance sheet at its fair value, and gains and losses that were accumulated in other comprehensive income will be recognized immediately in earnings. In all other situations in which hedge accounting is discontinued, the derivative will be carried at its fair value on the balance sheet, with subsequent changes in its fair value recognized in current-period earnings.</span></div><div style="margin-bottom:7pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">The Company also enters into certain interest rate swap contracts that are not designated as hedging instruments. These derivative contracts relate to transactions in which the Company enters into an interest rate swap with a client while at the same time entering into an offsetting interest rate swap with a third-party financial institution. Because the Company acts as an intermediary for the client, changes in the fair value of the underlying derivative contracts for the most part offset each other and do not significantly impact the Company’s results of operations. </span></div><div style="margin-bottom:8pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:118%">In the normal course of business, the Company enters into interest rate lock commitments to finance residential mortgage loans that are not designated as accounting hedges. These commitments, which contain fixed expiration dates, offer the borrower an interest rate guarantee provided the loan meets underwriting guidelines and closes within the timeframe established by the Company. Interest rate risk arises on these commitments and subsequently closed loans if interest rates change between the time of the interest rate lock and the delivery of the loan to the investor. Loan commitments related to residential mortgage loans intended to be sold are considered derivatives and are marked to market through earnings. In addition to the effects of the change in market interest rate, the fair value measurement of the derivative also contemplates the expected cash flows to be received from the counterparty from the future sale of the loan.</span></div>The Company sells residential mortgage loans on either a best efforts or mandatory delivery basis. The Company mitigates the effect of the interest rate risk inherent in providing interest rate lock commitments by entering into forward loan sales contracts. During the interest rate lock commitment period, these forward loan sales contracts are marked to market through earnings and are not designated as accounting hedges. Exclusive of the fair value component associated with the projected cash flows from the loan delivery to the investor, the changes in fair value related to movements in market rates of the interest rate lock commitments and the forward loan sales contracts generally move in opposite directions, and the net impact of changes in these valuations on net income during the loan commitment period is generally inconsequential. When the loan is funded to the borrower, the interest rate lock commitment derivative expires and the Company records a loan held for sale. The forward loan sales contract acts as a hedge against the variability in cash to be received from the loan sale. The changes in measurement of the estimated fair values of the interest rate lock commitments and forward loan sales contracts are included in mortgage banking revenues in the accompanying consolidated statements of income.
CY2022	us-gaap	Hosting Arrangement Service Contract Implementation Cost Expense Amortization HostingArrangementServiceContractImplementationCostExpenseAmortization	3400000	usd
CY2021	us-gaap	Hosting Arrangement Service Contract Implementation Cost Expense Amortization HostingArrangementServiceContractImplementationCostExpenseAmortization	4600000	usd
CY2022	us-gaap	Income Tax Examination Penalties Expense IncomeTaxExaminationPenaltiesExpense	0	usd
CY2021	us-gaap	Income Tax Examination Penalties Expense IncomeTaxExaminationPenaltiesExpense	0	usd
CY2020	us-gaap	Income Tax Examination Penalties Expense IncomeTaxExaminationPenaltiesExpense	0	usd
CY2022	us-gaap	Number Of Operating Segments NumberOfOperatingSegments	1	segment
CY2022	us-gaap	Advertising Expense AdvertisingExpense	3800000	usd
CY2021	us-gaap	Advertising Expense AdvertisingExpense	2600000	usd
CY2020	us-gaap	Advertising Expense AdvertisingExpense	2800000	usd
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Share Based Compensation Excess Tax Benefit Amount EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitAmount	200000	usd
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Share Based Compensation Excess Tax Benefit Amount EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitAmount	-500000	usd
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Share Based Compensation Excess Tax Benefit Amount EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitAmount	-400000	usd
CY2022	us-gaap	Goodwill Period Increase Decrease GoodwillPeriodIncreaseDecrease	-36700000	usd
CY2022	us-gaap	Goodwill Period Increase Decrease GoodwillPeriodIncreaseDecrease	-36700000	usd
CY2022Q1	fibk	Certain Loans Acquired In Transfer Accounted For As Available For Sale Debt Securities Accrued Interest CertainLoansAcquiredInTransferAccountedForAsAvailableForSaleDebtSecuritiesAccruedInterest	-18100000	usd
CY2022	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	118900000	usd
CY2021	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	11600000	usd
CY2022	us-gaap	Business Acquisition Pro Forma Earnings Per Share Basic BusinessAcquisitionProFormaEarningsPerShareBasic	3.28
CY2021	us-gaap	Business Acquisition Pro Forma Earnings Per Share Basic BusinessAcquisitionProFormaEarningsPerShareBasic	2.24
CY2022	us-gaap	Business Acquisition Pro Forma Earnings Per Share Diluted BusinessAcquisitionProFormaEarningsPerShareDiluted	3.28
CY2021	us-gaap	Business Acquisition Pro Forma Earnings Per Share Diluted BusinessAcquisitionProFormaEarningsPerShareDiluted	2.24
CY2021Q4	us-gaap	Goodwill Goodwill	621600000	usd
CY2020Q4	us-gaap	Goodwill Goodwill	621600000	usd
CY2022	us-gaap	Goodwill Acquired During Period GoodwillAcquiredDuringPeriod	479300000	usd
CY2021	us-gaap	Goodwill Acquired During Period GoodwillAcquiredDuringPeriod	0	usd
CY2022Q4	us-gaap	Goodwill Goodwill	1100900000	usd
CY2021Q4	us-gaap	Goodwill Goodwill	621600000	usd
CY2019Q3	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	0	usd
CY2018Q3	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	0	usd
CY2020Q3	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	0	usd
CY2021Q4	us-gaap	Finite Lived Intangible Assets Gross FiniteLivedIntangibleAssetsGross	106000000.0	usd
CY2022	us-gaap	Finitelived Intangible Assets Acquired1 FinitelivedIntangibleAssetsAcquired1	72900000	usd
CY2022	us-gaap	Finite Lived Intangible Assets Period Increase Decrease FiniteLivedIntangibleAssetsPeriodIncreaseDecrease	-1400000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	80500000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Net FiniteLivedIntangibleAssetsNet	97000000.0	usd
CY2020Q4	us-gaap	Finite Lived Intangible Assets Gross FiniteLivedIntangibleAssetsGross	106000000.0	usd
CY2021Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	64700000	usd
CY2021Q4	us-gaap	Finite Lived Intangible Assets Net FiniteLivedIntangibleAssetsNet	41300000	usd
CY2022	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	15900000	usd
CY2021	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	9900000	usd
CY2020	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	10900000	usd
CY2022	us-gaap	Acquired Finite Lived Intangible Assets Weighted Average Useful Life AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife	P12Y
CY2022Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Next Twelve Months FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths	15600000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Two FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo	14600000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Three FiniteLivedIntangibleAssetsAmortizationExpenseYearThree	13700000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Four FiniteLivedIntangibleAssetsAmortizationExpenseYearFour	12800000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Five FiniteLivedIntangibleAssetsAmortizationExpenseYearFive	10100000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense After Year Five FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive	30200000	usd
CY2022Q4	us-gaap	Finite Lived Intangible Assets Net FiniteLivedIntangibleAssetsNet	97000000.0	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	7573500000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	2300000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	629700000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	6946100000	usd
CY2022Q4	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	3453700000	usd
CY2022Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	1900000	usd
CY2022Q4	us-gaap	Debt Securities Held To Maturity Amortized Cost After Allowance For Credit Loss DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss	3451800000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Gain HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain	400000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Loss HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss	400000000.0	usd
CY2022Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	3052200000	usd
CY2022	fibk	Debt Securities Held To Maturity Transfer Unamortized Loss DebtSecuritiesHeldToMaturityTransferUnamortizedLoss	20200000	usd
CY2022	fibk	Debt Securities Held To Maturity Transfer Unamortized Gain DebtSecuritiesHeldToMaturityTransferUnamortizedGain	14200000	usd
CY2021Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	4859300000	usd
CY2021Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	20600000	usd
CY2021Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	59400000	usd
CY2021Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	4820500000	usd
CY2021Q4	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	1687600000	usd
CY2021Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	0	usd
CY2021Q4	us-gaap	Debt Securities Held To Maturity Amortized Cost After Allowance For Credit Loss DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss	1687600000	usd
CY2021Q4	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Gain HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain	15600000	usd
CY2021Q4	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Loss HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss	35700000	usd
CY2021Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	1667500000	usd
CY2021	fibk	Debt Securities Held To Maturity Transfer Unamortized Gain DebtSecuritiesHeldToMaturityTransferUnamortizedGain	20100000	usd
CY2022Q1	us-gaap	Debt Securities Held To Maturity Transfer Amount DebtSecuritiesHeldToMaturityTransferAmount	10700000	usd
CY2022Q1	fibk	Debt Securities Held To Maturity After Allowance For Credit Loss Transfer Amount DebtSecuritiesHeldToMaturityAfterAllowanceForCreditLossTransferAmount	10900000	usd
CY2022Q1	fibk	Debt Securities Available For Sale Amortized Cost After Allowance For Credit Loss Transfer Amount DebtSecuritiesAvailableForSaleAmortizedCostAfterAllowanceForCreditLossTransferAmount	485900000	usd
CY2022Q1	fibk	Debt Securities Available For Sale After Allowance For Credit Loss Transfer Amount DebtSecuritiesAvailableForSaleAfterAllowanceForCreditLossTransferAmount	463600000	usd
CY2022Q1	us-gaap	Held To Maturity Securities Transferred To Trading Securities Unrealized Gain Loss HeldToMaturitySecuritiesTransferredToTradingSecuritiesUnrealizedGainLoss	0	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months	5071900000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months Accumulated Loss DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	346600000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger	1545600000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer Accumulated Loss DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	283100000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position DebtSecuritiesAvailableForSaleUnrealizedLossPosition	6617500000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Accumulated Loss DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss	629700000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue	1923900000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Accumulated Loss HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	187200000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position Twelve Months Or Longer Fair Value HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue	1075200000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Accumulated Loss HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	212800000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position Fair Value HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue	2999100000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position Accumulated Loss HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss	400000000.0	usd
CY2021Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months	2760300000	usd
CY2021Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months Accumulated Loss DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	58300000	usd
CY2021Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger	68700000	usd
CY2021Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer Accumulated Loss DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	1100000	usd
CY2021Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position DebtSecuritiesAvailableForSaleUnrealizedLossPosition	2829000000	usd
CY2021Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Accumulated Loss DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss	59400000	usd
CY2021Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue	1067700000	usd
CY2021Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Accumulated Loss HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	35700000	usd
CY2021Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position Twelve Months Or Longer Fair Value HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue	0	usd
CY2021Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Accumulated Loss HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	0	usd
CY2021Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position Fair Value HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue	1067700000	usd
CY2021Q4	us-gaap	Held To Maturity Securities Continuous Unrealized Loss Position Accumulated Loss HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss	35700000	usd
CY2021Q4	us-gaap	Debt Securities Available For Sale Allowance For Credit Loss DebtSecuritiesAvailableForSaleAllowanceForCreditLoss	0	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Allowance For Credit Loss DebtSecuritiesAvailableForSaleAllowanceForCreditLoss	0	usd
CY2022Q4	fibk	Available For Sale And Held To Maturity Securities In Unrealized Loss Positions Qualitative Disclosure Number Of Positions AvailableForSaleAndHeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions	1222	investments
CY2021Q4	fibk	Available For Sale And Held To Maturity Securities In Unrealized Loss Positions Qualitative Disclosure Number Of Positions AvailableForSaleAndHeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions	285	investments
CY2021Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	0	usd
CY2020Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	0	usd
CY2019Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	0	usd
CY2022	us-gaap	Debt Securities Held To Maturity Credit Loss Expense Reversal DebtSecuritiesHeldToMaturityCreditLossExpenseReversal	1900000	usd
CY2021	us-gaap	Debt Securities Held To Maturity Credit Loss Expense Reversal DebtSecuritiesHeldToMaturityCreditLossExpenseReversal	0	usd
CY2020	us-gaap	Debt Securities Held To Maturity Credit Loss Expense Reversal DebtSecuritiesHeldToMaturityCreditLossExpenseReversal	0	usd
CY2022Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	1900000	usd
CY2021Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	0	usd
CY2020Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	0	usd
CY2022Q4	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	3453700000	usd
CY2021Q4	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	1687600000	usd
CY2022Q4	us-gaap	Accrued Investment Income Receivable AccruedInvestmentIncomeReceivable	38900000	usd
CY2021Q4	us-gaap	Accrued Investment Income Receivable AccruedInvestmentIncomeReceivable	16600000	usd
CY2022Q4	us-gaap	Debt Securities Held To Maturity Nonaccrual DebtSecuritiesHeldToMaturityNonaccrual	0	usd
CY2022	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	300000	usd
CY2022	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	-46800000	usd
CY2022	us-gaap	Gain Loss On Fair Value Hedges Recognized In Earnings GainLossOnFairValueHedgesRecognizedInEarnings	-46300000	usd
CY2021	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	3200000	usd
CY2021	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	-2100000	usd
CY2020	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	0	usd
CY2020	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	0	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities Next Rolling Twelve Months Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis	48800000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities Next Rolling Twelve Months Fair Value AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue	48100000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Debt Maturities Within One Year Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount	2500000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Debt Maturities Next Rolling Twelve Months Fair Value HeldToMaturitySecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue	2500000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Two Through Five Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis	1478900000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Two Through Five Fair Value AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue	1419800000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Debt Maturities After One Through Five Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount	607600000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Debt Maturities Rolling Year Two Through Five Fair Value HeldToMaturitySecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue	581600000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Six Through Ten Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis	1750000000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Six Through Ten Fair Value AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue	1564300000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Debt Maturities After Five Through Ten Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount	732700000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Debt Maturities Rolling Year Six Through Ten Fair Value HeldToMaturitySecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue	639500000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling After Year Ten Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenAmortizedCostBasis	4295800000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling After Year Ten Fair Value AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenFairValue	3913900000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Debt Maturities After Ten Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount	2110900000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Debt Maturities Rolling After Ten Years Fair Value HeldToMaturitySecuritiesDebtMaturitiesRollingAfterTenYearsFairValue	1828600000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	7573500000	usd
CY2022Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	6946100000	usd
CY2022Q4	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	3453700000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	3052200000	usd
CY2022Q4	fibk	Pledged Assets Separately Reported Securities Pledged For Repurchase Agreements At Cost PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtCost	4998900000	usd
CY2021Q4	fibk	Pledged Assets Separately Reported Securities Pledged For Repurchase Agreements At Cost PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtCost	2617800000	usd
CY2022Q4	us-gaap	Securities Loaned Or Sold Under Agreements To Repurchase Fair Value Disclosure SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure	4432000000	usd
CY2021Q4	us-gaap	Securities Loaned Or Sold Under Agreements To Repurchase Fair Value Disclosure SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure	2610800000	usd
CY2022Q4	us-gaap	Federal Home Loan Bank Stock And Federal Reserve Bank Stock FederalHomeLoanBankStockAndFederalReserveBankStock	198600000	usd
CY2021Q4	us-gaap	Federal Home Loan Bank Stock And Federal Reserve Bank Stock FederalHomeLoanBankStockAndFederalReserveBankStock	53800000	usd
CY2022Q4	us-gaap	Mortgages Held For Sale Fair Value Disclosure MortgagesHeldForSaleFairValueDisclosure	79900000	usd
CY2021Q4	us-gaap	Mortgages Held For Sale Fair Value Disclosure MortgagesHeldForSaleFairValueDisclosure	30100000	usd
CY2022Q4	us-gaap	Financing Receivable Recorded Investment Nonaccrual Status FinancingReceivableRecordedInvestmentNonaccrualStatus	39800000	usd
CY2022Q4	us-gaap	Loans And Leases Receivable Deferred Income LoansAndLeasesReceivableDeferredIncome	15600000	usd
CY2021Q4	us-gaap	Loans And Leases Receivable Deferred Income LoansAndLeasesReceivableDeferredIncome	12300000	usd
CY2022Q4	us-gaap	Loans And Leases Receivable Net Of Deferred Income LoansAndLeasesReceivableNetOfDeferredIncome	18099200000	usd
CY2021Q4	us-gaap	Loans And Leases Receivable Net Of Deferred Income LoansAndLeasesReceivableNetOfDeferredIncome	9331700000	usd
CY2022Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	220100000	usd
CY2021Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	122300000	usd
CY2022Q4	us-gaap	Loans And Leases Receivable Net Reported Amount LoansAndLeasesReceivableNetReportedAmount	17879100000	usd
CY2021Q4	us-gaap	Loans And Leases Receivable Net Reported Amount LoansAndLeasesReceivableNetReportedAmount	9209400000	usd
CY2021Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	122300000	usd
CY2022	us-gaap	Financing Receivable Purchased With Credit Deterioration Allowance For Credit Loss At Acquisition Date FinancingReceivablePurchasedWithCreditDeteriorationAllowanceForCreditLossAtAcquisitionDate	59500000	usd
CY2022	us-gaap	Off Balance Sheet Credit Loss Liability Credit Loss Expense Reversal OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal	68400000	usd
CY2022	us-gaap	Allowance For Loan And Lease Losses Write Offs AllowanceForLoanAndLeaseLossesWriteOffs	45000000.0	usd
CY2022	us-gaap	Financing Receivable Allowance For Credit Losses Recovery FinancingReceivableAllowanceForCreditLossesRecovery	14900000	usd
CY2022Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	220100000	usd
CY2020Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	144300000	usd
CY2021	us-gaap	Off Balance Sheet Credit Loss Liability Credit Loss Expense Reversal OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal	-14700000	usd
CY2021	us-gaap	Allowance For Loan And Lease Losses Write Offs AllowanceForLoanAndLeaseLossesWriteOffs	16600000	usd
CY2021	us-gaap	Financing Receivable Allowance For Credit Losses Recovery FinancingReceivableAllowanceForCreditLossesRecovery	9300000	usd
CY2021Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	122300000	usd
CY2019Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	73000000.0	usd
CY2020	us-gaap	Financing Receivable Allowance For Credit Losses Effect Of Change In Method FinancingReceivableAllowanceForCreditLossesEffectOfChangeInMethod	30000000.0	usd
CY2020	us-gaap	Off Balance Sheet Credit Loss Liability Credit Loss Expense Reversal OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal	55500000	usd
CY2020	us-gaap	Allowance For Loan And Lease Losses Write Offs AllowanceForLoanAndLeaseLossesWriteOffs	20900000	usd
CY2020	us-gaap	Financing Receivable Allowance For Credit Losses Recovery FinancingReceivableAllowanceForCreditLossesRecovery	6700000	usd
CY2020Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	144300000	usd
CY2022Q4	us-gaap	Loans And Leases Receivable Impaired Nonperforming Accrual Of Interest LoansAndLeasesReceivableImpairedNonperformingAccrualOfInterest	0	usd
CY2022	us-gaap	Impaired Financing Receivable Interest Income Accrual Method ImpairedFinancingReceivableInterestIncomeAccrualMethod	0	usd
CY2022Q4	us-gaap	Financing Receivable Modifications Recorded Investment FinancingReceivableModificationsRecordedInvestment	64600000	usd
CY2022Q4	fibk	Financing Receivable Modifications Recorded Investment Non Accrual FinancingReceivableModificationsRecordedInvestmentNonAccrual	4200000	usd
CY2022Q4	fibk	Financing Receivable Modifications Recorded Investment Accrual Method FinancingReceivableModificationsRecordedInvestmentAccrualMethod	60400000	usd
CY2022Q4	us-gaap	Long Term Accounts Notes And Loans Receivable Net Noncurrent LongTermAccountsNotesAndLoansReceivableNetNoncurrent	1100000	usd
CY2022Q4	us-gaap	Loans And Leases Receivable Impaired Commitment To Lend LoansAndLeasesReceivableImpairedCommitmentToLend	0	usd
CY2021Q4	us-gaap	Financing Receivable Modifications Recorded Investment FinancingReceivableModificationsRecordedInvestment	6200000	usd
CY2021Q4	fibk	Financing Receivable Modifications Recorded Investment Non Accrual FinancingReceivableModificationsRecordedInvestmentNonAccrual	3900000	usd
CY2021Q4	fibk	Financing Receivable Modifications Recorded Investment Accrual Method FinancingReceivableModificationsRecordedInvestmentAccrualMethod	2300000	usd
CY2022Q4	us-gaap	Allowance For Notes And Loans Receivable Noncurrent AllowanceForNotesAndLoansReceivableNoncurrent	100000	usd
CY2022Q4	us-gaap	Loans And Leases Receivable Impaired Commitment To Lend LoansAndLeasesReceivableImpairedCommitmentToLend	0	usd
CY2022	us-gaap	Financing Receivable Modifications Post Modification Recorded Investment2 FinancingReceivableModificationsPostModificationRecordedInvestment2	71700000	usd
CY2022	us-gaap	Financing Receivable Modifications Number Of Contracts2 FinancingReceivableModificationsNumberOfContracts2	12	note
CY2022	us-gaap	Financing Receivable Modifications Post Modification Recorded Investment2 FinancingReceivableModificationsPostModificationRecordedInvestment2	71700000	usd
CY2022	us-gaap	Financing Receivables Impaired Troubled Debt Restructuring Write Down FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown	5700000	usd
CY2021	us-gaap	Financing Receivables Impaired Troubled Debt Restructuring Write Down FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown	0	usd
CY2020	us-gaap	Financing Receivables Impaired Troubled Debt Restructuring Write Down FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown	0	usd
CY2022	us-gaap	Financing Receivable Modifications Subsequent Default Recorded Investment1 FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1	0	usd
CY2022Q4	us-gaap	Impaired Financing Receivable Recorded Investment ImpairedFinancingReceivableRecordedInvestment	743300000	usd
CY2022Q1	us-gaap	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Acquired During Period Cash Flows Expected To Be Collected At Acquisition CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodCashFlowsExpectedToBeCollectedAtAcquisition	623300000	usd
CY2022Q1	us-gaap	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Allowance For Loan Losses CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses	298200000	usd
CY2022Q1	fibk	Certain Loans And Debt Securities Acquired In Transfer Discount At Acquisition CertainLoansAndDebtSecuritiesAcquiredInTransferDiscountAtAcquisition	57700000	usd
CY2022Q1	fibk	Certain Loans And Debt Securities Acquired In Transfer Part Value At Acquisition CertainLoansAndDebtSecuritiesAcquiredInTransferPartValueAtAcquisition	979200000	usd
CY2022Q1	us-gaap	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Allowance For Loan Losses CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses	298200000	usd
CY2022Q1	us-gaap	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Allowance For Loan Losses Decreases1 CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLossesDecreases1	238700000	usd
CY2022Q1	fibk	Business Combination Impact On Purchased Credit Deteriorated Loans BusinessCombinationImpactOnPurchasedCreditDeterioratedLoans	59500000	usd
CY2022Q1	fibk	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Difference Between Amortized Cost Basis And Unpaid Principal Balance CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesDifferenceBetweenAmortizedCostBasisAndUnpaidPrincipalBalance	39600000	usd
CY2022Q1	fibk	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Interest Applied To Principal CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesInterestAppliedToPrincipal	18100000	usd
CY2022	us-gaap	Loans And Leases Receivable Commitments To Purchase Or Sell LoansAndLeasesReceivableCommitmentsToPurchaseOrSell	0	usd
CY2022	us-gaap	Proceeds From Sale Of Loans And Leases Held For Investment ProceedsFromSaleOfLoansAndLeasesHeldForInvestment	0	usd
CY2022Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	666900000	usd
CY2021Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	495800000	usd
CY2022Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	222200000	usd
CY2021Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	196200000	usd
CY2022Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	444700000	usd
CY2021Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	299600000	usd
CY2022	us-gaap	Depreciation Depreciation	35100000	usd
CY2021	us-gaap	Depreciation Depreciation	28000000	usd
CY2020	us-gaap	Depreciation Depreciation	25800000	usd
CY2022Q4	us-gaap	Life Insurance Corporate Or Bank Owned Amount LifeInsuranceCorporateOrBankOwnedAmount	497900000	usd
CY2021Q4	us-gaap	Life Insurance Corporate Or Bank Owned Amount LifeInsuranceCorporateOrBankOwnedAmount	301500000	usd
CY2021Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	2000000.0	usd
CY2020Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	2500000	usd
CY2019Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	8500000	usd
CY2022	fibk	Real Estate Acquired Through Foreclosure Acquired Through Acquisition RealEstateAcquiredThroughForeclosureAcquiredThroughAcquisition	15800000	usd
CY2021	fibk	Real Estate Acquired Through Foreclosure Acquired Through Acquisition RealEstateAcquiredThroughForeclosureAcquiredThroughAcquisition	0	usd
CY2020	fibk	Real Estate Acquired Through Foreclosure Acquired Through Acquisition RealEstateAcquiredThroughForeclosureAcquiredThroughAcquisition	0	usd
CY2022	fibk	Real Estate Acquired Through Foreclosure Additions RealEstateAcquiredThroughForeclosureAdditions	400000	usd
CY2021	fibk	Real Estate Acquired Through Foreclosure Additions RealEstateAcquiredThroughForeclosureAdditions	900000	usd
CY2020	fibk	Real Estate Acquired Through Foreclosure Additions RealEstateAcquiredThroughForeclosureAdditions	3300000	usd
CY2022	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	2800000	usd
CY2021	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	0	usd
CY2020	us-gaap	Impairment Of Real Estate ImpairmentOfRealEstate	100000	usd
CY2022	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	2700000	usd
CY2021	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	1400000	usd
CY2020	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	9200000	usd
CY2022Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	12700000	usd
CY2021Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	2000000.0	usd
CY2020Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	2500000	usd
CY2022	us-gaap	Impairment Of Real Estate ImpairmentOfRealEstate	2800000	usd
CY2021	us-gaap	Impairment Of Real Estate ImpairmentOfRealEstate	0	usd
CY2022Q4	fibk	Foreclosed Residential Real Estateincludedin Other Real Estate Owned ForeclosedResidentialRealEstateincludedinOtherRealEstateOwned	0	usd
CY2022Q4	us-gaap	Mortgage Loans In Process Of Foreclosure Amount MortgageLoansInProcessOfForeclosureAmount	0	usd
CY2022Q4	us-gaap	Interest Rate Cash Flow Hedge Gain Loss To Be Reclassified During Next12 Months Net InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet	5400000	usd
CY2022Q4	us-gaap	Derivative Instruments And Hedges DerivativeInstrumentsAndHedges	191900000	usd
CY2022Q4	us-gaap	Hedged Asset Fair Value Hedge Cumulative Increase Decrease HedgedAssetFairValueHedgeCumulativeIncreaseDecrease	8100000	usd
CY2021Q4	us-gaap	Derivative Instruments And Hedges DerivativeInstrumentsAndHedges	695600000	usd
CY2021Q4	us-gaap	Hedged Asset Fair Value Hedge Cumulative Increase Decrease HedgedAssetFairValueHedgeCumulativeIncreaseDecrease	-4400000	usd
CY2022Q4	us-gaap	Derivative Asset Notional Amount DerivativeAssetNotionalAmount	2290700000	usd
CY2022Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	45200000	usd
CY2021Q4	us-gaap	Derivative Asset Notional Amount DerivativeAssetNotionalAmount	1691200000	usd
CY2021Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	28100000	usd
CY2022Q4	us-gaap	Derivative Liability Notional Amount DerivativeLiabilityNotionalAmount	2449000000	usd
CY2022Q4	us-gaap	Interest Rate Derivative Liabilities At Fair Value InterestRateDerivativeLiabilitiesAtFairValue	159600000	usd
CY2021Q4	us-gaap	Derivative Liability Notional Amount DerivativeLiabilityNotionalAmount	1103900000	usd
CY2021Q4	us-gaap	Interest Rate Derivative Liabilities At Fair Value InterestRateDerivativeLiabilitiesAtFairValue	18200000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationTax	-2200000	usd
CY2022	us-gaap	Unrealized Gain Loss On Interest Rate Cash Flow Hedges Pretax Accumulated Other Comprehensive Income Loss UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss	0	usd
CY2021	us-gaap	Unrealized Gain Loss On Interest Rate Cash Flow Hedges Pretax Accumulated Other Comprehensive Income Loss UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss	0	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	45200000	usd
CY2022Q4	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	usd
CY2022Q4	us-gaap	Derivative Assets DerivativeAssets	45200000	usd
CY2022Q4	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	usd
CY2022Q4	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	45100000	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	100000	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	45200000	usd
CY2022Q4	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	usd
CY2022Q4	us-gaap	Derivative Assets DerivativeAssets	45200000	usd
CY2022Q4	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	usd
CY2022Q4	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	45100000	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	100000	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Liability DerivativeFairValueOfDerivativeLiability	159600000	usd
CY2022Q4	us-gaap	Derivative Liability Fair Value Gross Asset DerivativeLiabilityFairValueGrossAsset	0	usd
CY2022Q4	us-gaap	Derivative Liabilities DerivativeLiabilities	159600000	usd
CY2022Q4	us-gaap	Derivative Collateral Right To Reclaim Securities DerivativeCollateralRightToReclaimSecurities	0	usd
CY2022Q4	us-gaap	Derivative Collateral Right To Reclaim Cash DerivativeCollateralRightToReclaimCash	0	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Liability Amount Offset Against Collateral DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral	159600000	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase Gross SecuritiesSoldUnderAgreementsToRepurchaseGross	1052900000	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase Asset SecuritiesSoldUnderAgreementsToRepurchaseAsset	0	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase SecuritiesSoldUnderAgreementsToRepurchase	1052900000	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Securities SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimSecurities	0	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Cash SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimCash	1052900000	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase Amount Offset Against Collateral SecuritiesSoldUnderAgreementsToRepurchaseAmountOffsetAgainstCollateral	0	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Gross DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedGross	1212500000	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Asset DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset	0	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoaned	1212500000	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Securities DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimSecurities	0	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Cash DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimCash	1052900000	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Amount Offset Against Collateral DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAmountOffsetAgainstCollateral	159600000	usd
CY2021Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	28100000	usd
CY2021Q4	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	usd
CY2021Q4	us-gaap	Derivative Assets DerivativeAssets	28100000	usd
CY2021Q4	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	usd
CY2021Q4	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	8000000.0	usd
CY2021Q4	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	20100000	usd
CY2021Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	28100000	usd
CY2021Q4	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	usd
CY2021Q4	us-gaap	Derivative Assets DerivativeAssets	28100000	usd
CY2021Q4	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	usd
CY2021Q4	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	8000000.0	usd
CY2021Q4	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	20100000	usd
CY2021Q4	us-gaap	Derivative Fair Value Of Derivative Liability DerivativeFairValueOfDerivativeLiability	18200000	usd
CY2021Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Asset DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset	0	usd
CY2021Q4	us-gaap	Derivative Liability Fair Value Gross Asset DerivativeLiabilityFairValueGrossAsset	0	usd
CY2021Q4	us-gaap	Derivative Liabilities DerivativeLiabilities	18200000	usd
CY2021Q4	us-gaap	Derivative Collateral Right To Reclaim Securities DerivativeCollateralRightToReclaimSecurities	0	usd
CY2021Q4	us-gaap	Derivative Collateral Right To Reclaim Cash DerivativeCollateralRightToReclaimCash	0	usd
CY2021Q4	us-gaap	Derivative Fair Value Of Derivative Liability Amount Offset Against Collateral DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral	18200000	usd
CY2021Q4	us-gaap	Securities Sold Under Agreements To Repurchase Gross SecuritiesSoldUnderAgreementsToRepurchaseGross	1051100000	usd
CY2021Q4	us-gaap	Securities Sold Under Agreements To Repurchase Asset SecuritiesSoldUnderAgreementsToRepurchaseAsset	0	usd
CY2021Q4	us-gaap	Securities Sold Under Agreements To Repurchase SecuritiesSoldUnderAgreementsToRepurchase	1051100000	usd
CY2021Q4	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Securities SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimSecurities	0	usd
CY2021Q4	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Cash SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimCash	1051100000	usd
CY2021Q4	us-gaap	Securities Sold Under Agreements To Repurchase Amount Offset Against Collateral SecuritiesSoldUnderAgreementsToRepurchaseAmountOffsetAgainstCollateral	0	usd
CY2021Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Gross DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedGross	1069300000	usd
CY2021Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoaned	1069300000	usd
CY2021Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Securities DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimSecurities	0	usd
CY2021Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Cash DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimCash	1051100000	usd
CY2021Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Amount Offset Against Collateral DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAmountOffsetAgainstCollateral	18200000	usd
CY2022Q4	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	7560000000	usd
CY2021Q4	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	5568300000	usd
CY2022Q4	us-gaap	Demand Deposit Accounts DemandDepositAccounts	7205900000	usd
CY2021Q4	us-gaap	Demand Deposit Accounts DemandDepositAccounts	4753200000	usd
CY2022Q4	us-gaap	Deposits Savings Deposits DepositsSavingsDeposits	8379300000	usd
CY2021Q4	us-gaap	Deposits Savings Deposits DepositsSavingsDeposits	4981600000	usd
CY2022Q4	us-gaap	Time Deposits100000 Or More TimeDeposits100000OrMore	438000000.0	usd
CY2021Q4	us-gaap	Time Deposits100000 Or More TimeDeposits100000OrMore	186700000	usd
CY2022Q4	us-gaap	Time Deposits Less Than100000 TimeDepositsLessThan100000	1490400000	usd
CY2021Q4	us-gaap	Time Deposits Less Than100000 TimeDepositsLessThan100000	779800000	usd
CY2022Q4	us-gaap	Interest Bearing Deposit Liabilities InterestBearingDepositLiabilities	17513600000	usd
CY2021Q4	us-gaap	Interest Bearing Deposit Liabilities InterestBearingDepositLiabilities	10701300000	usd
CY2022Q4	us-gaap	Deposits Deposits	25073600000	usd
CY2021Q4	us-gaap	Deposits Deposits	16269600000	usd
CY2022Q4	fibk	Fdic Deposit Insurance Limit FDICDepositInsuranceLimit	250000000	usd
CY2022Q4	fibk	Time Deposits Obtained Certificate Of Deposit Account Registry Service TimeDepositsObtainedCertificateOfDepositAccountRegistryService	36600000	usd
CY2021Q4	fibk	Time Deposits Obtained Certificate Of Deposit Account Registry Service TimeDepositsObtainedCertificateOfDepositAccountRegistryService	104500000	usd
CY2022Q4	us-gaap	Interest Bearing Domestic Deposit Brokered InterestBearingDomesticDepositBrokered	0	usd
CY2022Q4	fibk	Contractual Maturities Time Deposits100000or Moreafter3years ContractualMaturitiesTimeDeposits100000orMoreafter3years	4800000	usd
CY2022Q4	fibk	Time Deposits250000or More TimeDeposits250000orMore	438000000	usd
CY2021Q4	fibk	Time Deposits250000or More TimeDeposits250000orMore	186700000	usd
CY2022Q4	fibk	Fdic Deposit Insurance Limit FDICDepositInsuranceLimit	250000000	usd
CY2022Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More Three Months Or Less ContractualMaturitiesTimeDeposits100000OrMoreThreeMonthsOrLess	47800000	usd
CY2022Q4	fibk	Contractual Maturities Time Deposits Three Monthsor Less ContractualMaturitiesTimeDepositsThreeMonthsorLess	311800000	usd
CY2022Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More Three Months Through Six Months ContractualMaturitiesTimeDeposits100000OrMoreThreeMonthsThroughSixMonths	34600000	usd
CY2022Q4	fibk	Contractual Maturities Time Deposits Three Monthsthrough Six Months ContractualMaturitiesTimeDepositsThreeMonthsthroughSixMonths	215400000	usd
CY2022Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More Six Months Through12 Months ContractualMaturitiesTimeDeposits100000OrMoreSixMonthsThrough12Months	283900000	usd
CY2022Q4	fibk	Contractual Maturities Time Deposits Six Monthsthrough Twelve Months ContractualMaturitiesTimeDepositsSixMonthsthroughTwelveMonths	1005400000	usd
CY2022Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More After12 Months ContractualMaturitiesTimeDeposits100000OrMoreAfter12Months	55000000.0	usd
CY2022Q4	us-gaap	Time Deposits Noncurrent TimeDepositsNoncurrent	282800000	usd
CY2022Q4	fibk	Contractual Maturities Time Deposits100000or Moreafter2years ContractualMaturitiesTimeDeposits100000orMoreafter2years	10700000	usd
CY2022Q4	us-gaap	Time Deposit Maturities Year Two TimeDepositMaturitiesYearTwo	69200000	usd
CY2022Q4	us-gaap	Time Deposit Maturities Year Three TimeDepositMaturitiesYearThree	27800000	usd
CY2022Q4	fibk	Contractual Maturities Time Deposits100000 Or More After4 Years ContractualMaturitiesTimeDeposits100000OrMoreAfter4Years	1200000	usd
CY2022Q4	us-gaap	Time Deposit Maturities After Year Five TimeDepositMaturitiesAfterYearFive	16000000.0	usd
CY2022Q4	us-gaap	Time Deposits100000 Or More TimeDeposits100000OrMore	438000000.0	usd
CY2022Q4	us-gaap	Time Deposits TimeDeposits	1928400000	usd
CY2022	us-gaap	Interest Expense Time Deposits100000 Or More InterestExpenseTimeDeposits100000OrMore	2500000	usd
CY2021	us-gaap	Interest Expense Time Deposits100000 Or More InterestExpenseTimeDeposits100000OrMore	900000	usd
CY2020	us-gaap	Interest Expense Time Deposits100000 Or More InterestExpenseTimeDeposits100000OrMore	2700000	usd
CY2020Q2	us-gaap	Debt Instrument Interest Rate Stated Percentage DebtInstrumentInterestRateStatedPercentage	0.0525
CY2022Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	120800000	usd
CY2021Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	112400000	usd
CY2022Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	100000	usd
CY2022Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	100000	usd
CY2022Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	100000	usd
CY2022Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Four LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour	100000	usd
CY2022Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive	200000	usd
CY2022Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal After Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive	120200000	usd
CY2022Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	120800000	usd
CY2020Q2	us-gaap	Debt Instrument Interest Rate Stated Percentage DebtInstrumentInterestRateStatedPercentage	0.0525
CY2022Q4	us-gaap	Debt Instrument Unamortized Discount Premium And Debt Issuance Costs Net DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet	1100000	usd
CY2022Q4	us-gaap	Advances From Federal Home Loan Banks AdvancesFromFederalHomeLoanBanks	2327000000	usd
CY2021Q4	us-gaap	Advances From Federal Home Loan Banks AdvancesFromFederalHomeLoanBanks	0	usd
CY2022Q4	us-gaap	Federal Home Loan Bank Advances FederalHomeLoanBankAdvances	0	usd
CY2022Q4	us-gaap	Other Borrowings OtherBorrowings	0	usd
CY2022	fibk	Company Sponsored Wholly Owned Business Trusts Number CompanySponsoredWhollyOwnedBusinessTrustsNumber	14	trusts
CY2022Q4	us-gaap	Subordinated Long Term Debt SubordinatedLongTermDebt	163100000	usd
CY2022Q4	us-gaap	Subordinated Debt SubordinatedDebt	163700000	usd
CY2022Q4	fibk	Common Stock Held In Assets CommonStockHeldInAssets	5300000	usd
CY2021Q4	us-gaap	Subordinated Debt SubordinatedDebt	87700000	usd
CY2021Q4	fibk	Common Stock Held In Assets CommonStockHeldInAssets	2700000	usd
CY2022Q4	fibk	Subordinated Debt Fair Value Adjustment SubordinatedDebtFairValueAdjustment	600000	usd
CY2021Q4	fibk	Subordinated Debt Fair Value Adjustment SubordinatedDebtFairValueAdjustment	700000	usd
CY2022Q4	us-gaap	Subordinated Long Term Debt SubordinatedLongTermDebt	163100000	usd
CY2021Q4	us-gaap	Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust	87000000.0	usd
CY2022Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	250000000	shares
CY2022Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	104442023	shares
CY2022Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	104442023	shares
CY2022	us-gaap	Accelerated Share Repurchases Adjustment To Recorded Amount AcceleratedShareRepurchasesAdjustmentToRecordedAmount	197400000	usd
CY2022	us-gaap	Treasury Stock Acquired Average Cost Per Share TreasuryStockAcquiredAverageCostPerShare	39.48
CY2021Q4	fibk	Sale Of Stock Shares Authorized For Future Sale SaleOfStockSharesAuthorizedForFutureSale	47158390	shares
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	usd
CY2020	us-gaap	Net Income Loss NetIncomeLoss	161200000	usd
CY2022	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	103274070	shares
CY2021	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	61650312	shares
CY2020	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	63611891	shares
CY2022	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	66929	shares
CY2021	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	91516	shares
CY2020	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	117579	shares
CY2022	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	103340999	shares
CY2021	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	61741828	shares
CY2020	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	63729470	shares
CY2022	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.96
CY2021	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	3.12
CY2020	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.53
CY2022	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.96
CY2021	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	3.11
CY2020	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.53
CY2022Q4	us-gaap	Capital Capital	2875800000	usd
CY2022Q4	us-gaap	Capital To Risk Weighted Assets CapitalToRiskWeightedAssets	0.1248
CY2022Q4	us-gaap	Capital Required For Capital Adequacy CapitalRequiredForCapitalAdequacy	1843200000	usd
CY2022Q4	us-gaap	Capital Required For Capital Adequacy To Risk Weighted Assets CapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0800
CY2022Q4	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	2419200000	usd
CY2022Q4	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.1050
CY2022Q4	us-gaap	Capital Required To Be Well Capitalized CapitalRequiredToBeWellCapitalized	2304000000	usd
CY2022Q4	us-gaap	Capital Required To Be Well Capitalized To Risk Weighted Assets CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.1000
CY2022Q4	us-gaap	Tier One Risk Based Capital TierOneRiskBasedCapital	2408800000	usd
CY2022Q4	us-gaap	Tier One Risk Based Capital To Risk Weighted Assets TierOneRiskBasedCapitalToRiskWeightedAssets	0.1045
CY2022Q4	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy TierOneRiskBasedCapitalRequiredForCapitalAdequacy	1382400000	usd
CY2022Q4	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0600
CY2022Q4	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	1958400000	usd
CY2022Q4	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0850
CY2022Q4	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized TierOneRiskBasedCapitalRequiredToBeWellCapitalized	1843200000	usd
CY2022Q4	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.0800
CY2022Q4	fibk	Common Equity Tier One Risked Based Capital CommonEquityTierOneRiskedBasedCapital	2408800000	usd
CY2022Q4	fibk	Common Equity Tier One Risk Based Capital To Risk Weighted Assets CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets	0.1045
CY2022Q4	fibk	Common Equity Tier One Capital Required For Capital Adequacy Under Basel Iii Phase In Schedule CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInSchedule	1036800000	usd
CY2022Q4	fibk	Common Equity Tier One Capital Required For Capital Adequacy Under Basel Iii Phase In Schedule To Risk Weighted Assets CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInScheduleToRiskWeightedAssets	0.0450
CY2022Q4	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	1612800000	usd
CY2022Q4	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0700
CY2022Q4	fibk	Common Equity Tier One Capital To Be Well Capitalized CommonEquityTierOneCapitalToBeWellCapitalized	1497600000	usd
CY2022Q4	fibk	Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets CommonEquityTierOneCapitalToBeWellCapitalizedToRiskWeightedAssets	0.0650
CY2022Q4	us-gaap	Tier One Leverage Capital TierOneLeverageCapital	2408800000	usd
CY2022Q4	us-gaap	Tier One Leverage Capital To Average Assets TierOneLeverageCapitalToAverageAssets	0.0775
CY2022Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	1242900000	usd
CY2022Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2022Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	1242900000	usd
CY2022Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2022Q4	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized TierOneLeverageCapitalRequiredToBeWellCapitalized	1553600000	usd
CY2022Q4	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized To Average Assets TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets	0.0500
CY2021Q4	us-gaap	Capital Capital	1659300000	usd
CY2021Q4	us-gaap	Capital To Risk Weighted Assets CapitalToRiskWeightedAssets	0.1411
CY2021Q4	us-gaap	Capital Required For Capital Adequacy CapitalRequiredForCapitalAdequacy	940900000	usd
CY2021Q4	us-gaap	Capital Required For Capital Adequacy To Risk Weighted Assets CapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0800
CY2021Q4	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	1235000000	usd
CY2021Q4	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.1050
CY2021Q4	us-gaap	Capital Required To Be Well Capitalized CapitalRequiredToBeWellCapitalized	1176200000	usd
CY2021Q4	us-gaap	Capital Required To Be Well Capitalized To Risk Weighted Assets CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.1000
CY2021Q4	us-gaap	Tier One Risk Based Capital TierOneRiskBasedCapital	1469000000	usd
CY2021Q4	us-gaap	Tier One Risk Based Capital To Risk Weighted Assets TierOneRiskBasedCapitalToRiskWeightedAssets	0.1249
CY2021Q4	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy TierOneRiskBasedCapitalRequiredForCapitalAdequacy	705700000	usd
CY2021Q4	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0600
CY2021Q4	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	999700000	usd
CY2021Q4	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0850
CY2021Q4	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized TierOneRiskBasedCapitalRequiredToBeWellCapitalized	940900000	usd
CY2021Q4	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.0800
CY2021Q4	fibk	Common Equity Tier One Risked Based Capital CommonEquityTierOneRiskedBasedCapital	1384800000	usd
CY2021Q4	fibk	Common Equity Tier One Risk Based Capital To Risk Weighted Assets CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets	0.1177
CY2021Q4	fibk	Common Equity Tier One Capital Required For Capital Adequacy Under Basel Iii Phase In Schedule CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInSchedule	529300000	usd
CY2021Q4	fibk	Common Equity Tier One Capital Required For Capital Adequacy Under Basel Iii Phase In Schedule To Risk Weighted Assets CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInScheduleToRiskWeightedAssets	0.0450
CY2021Q4	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	823300000	usd
CY2021Q4	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0700
CY2021Q4	fibk	Common Equity Tier One Capital To Be Well Capitalized CommonEquityTierOneCapitalToBeWellCapitalized	764500000	usd
CY2021Q4	fibk	Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets CommonEquityTierOneCapitalToBeWellCapitalizedToRiskWeightedAssets	0.0650
CY2021Q4	us-gaap	Tier One Leverage Capital TierOneLeverageCapital	1469000000	usd
CY2021Q4	us-gaap	Tier One Leverage Capital To Average Assets TierOneLeverageCapitalToAverageAssets	0.0768
CY2021Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	765500000	usd
CY2021Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2021Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	765500000	usd
CY2021Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2021Q4	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized TierOneLeverageCapitalRequiredToBeWellCapitalized	956900000	usd
CY2021Q4	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized To Average Assets TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets	0.0500
CY2022	us-gaap	Operating Lease Expense OperatingLeaseExpense	8900000	usd
CY2021	us-gaap	Operating Lease Expense OperatingLeaseExpense	4300000	usd
CY2020	us-gaap	Lease And Rental Expense LeaseAndRentalExpense	5100000	usd
CY2022Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	53900000	usd
CY2021Q4	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	3800000	usd
CY2020Q4	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	3700000	usd
CY2022Q4	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	16200000	usd
CY2021Q4	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	3800000	usd
CY2021Q4	fibk	Credit Extension Commitments Beyond One Year CreditExtensionCommitmentsBeyondOneYear	2539800000	usd
CY2022	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	45200000	usd
CY2021	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	39400000	usd
CY2020	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	42400000	usd
CY2022	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	14400000	usd
CY2021	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	11300000	usd
CY2020	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	12300000	usd
CY2022	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	59600000	usd
CY2021	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	50700000	usd
CY2020	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	54700000	usd
CY2022	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-3700000	usd
CY2021	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	3700000	usd
CY2020	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-5700000	usd
CY2022	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-1000000.0	usd
CY2021	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	1300000	usd
CY2020	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-900000	usd
CY2022	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-4700000	usd
CY2021	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	5000000.0	usd
CY2020	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-6600000	usd
CY2022	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	54900000	usd
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	55700000	usd
CY2020	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	48100000	usd
CY2022	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	54000000.0	usd
CY2021	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	52000000.0	usd
CY2020	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	44000000.0	usd
CY2022	us-gaap	Income Tax Reconciliation Tax Exempt Income IncomeTaxReconciliationTaxExemptIncome	8400000	usd
CY2021	us-gaap	Income Tax Reconciliation Tax Exempt Income IncomeTaxReconciliationTaxExemptIncome	2800000	usd
CY2020	us-gaap	Income Tax Reconciliation Tax Exempt Income IncomeTaxReconciliationTaxExemptIncome	2100000	usd
CY2022	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	10600000	usd
CY2021	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	9900000	usd
CY2020	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	9000000.0	usd
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Share Based Compensation Excess Tax Benefit Amount EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitAmount	200000	usd
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Share Based Compensation Excess Tax Benefit Amount EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitAmount	-500000	usd
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Share Based Compensation Excess Tax Benefit Amount EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitAmount	-400000	usd
CY2022	us-gaap	Income Tax Reconciliation Nondeductible Expense IncomeTaxReconciliationNondeductibleExpense	2000000.0	usd
CY2021	us-gaap	Income Tax Reconciliation Nondeductible Expense IncomeTaxReconciliationNondeductibleExpense	800000	usd
CY2020	us-gaap	Income Tax Reconciliation Nondeductible Expense IncomeTaxReconciliationNondeductibleExpense	0	usd
CY2022	us-gaap	Income Tax Credits And Adjustments IncomeTaxCreditsAndAdjustments	-4300000	usd
CY2021	us-gaap	Income Tax Credits And Adjustments IncomeTaxCreditsAndAdjustments	-4300000	usd
CY2020	us-gaap	Income Tax Credits And Adjustments IncomeTaxCreditsAndAdjustments	-2300000	usd
CY2022	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	800000	usd
CY2021	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	600000	usd
CY2020	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	-100000	usd
CY2022	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	54900000	usd
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	55700000	usd
CY2020	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	48100000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Provision For Loan Losses DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsProvisionForLoanLosses	54800000	usd
CY2021Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Provision For Loan Losses DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsProvisionForLoanLosses	30700000	usd
CY2022Q4	fibk	Deferredtaxassetloandiscount Deferredtaxassetloandiscount	32500000	usd
CY2021Q4	fibk	Deferredtaxassetloandiscount Deferredtaxassetloandiscount	1600000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Unrealized Losses On Availablefor Sale Securities Gross DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross	157900000	usd
CY2021Q4	us-gaap	Deferred Tax Assets Unrealized Losses On Availablefor Sale Securities Gross DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross	4600000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Derivative Instruments DeferredTaxAssetsDerivativeInstruments	500000	usd
CY2021Q4	us-gaap	Deferred Tax Assets Derivative Instruments DeferredTaxAssetsDerivativeInstruments	0	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Employee Benefits DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits	26800000	usd
CY2021Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Employee Benefits DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits	19300000	usd
CY2022Q4	fibk	Deferred Tax Assets Non Performing Loan Interest DeferredTaxAssetsNonPerformingLoanInterest	2600000	usd
CY2021Q4	fibk	Deferred Tax Assets Non Performing Loan Interest DeferredTaxAssetsNonPerformingLoanInterest	1000000.0	usd
CY2022Q4	fibk	Deferred Tax Assets Other Real Estate Owned Write Downs And Carrying Costs DeferredTaxAssetsOtherRealEstateOwnedWriteDownsAndCarryingCosts	1500000	usd
CY2021Q4	fibk	Deferred Tax Assets Other Real Estate Owned Write Downs And Carrying Costs DeferredTaxAssetsOtherRealEstateOwnedWriteDownsAndCarryingCosts	0	usd
CY2022Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	1500000	usd
CY2021Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	1700000	usd
CY2022Q4	fibk	Deferred Tax Liability Lease Liabilities DeferredTaxLiabilityLeaseLiabilities	12000000.0	usd
CY2021Q4	fibk	Deferred Tax Liability Lease Liabilities DeferredTaxLiabilityLeaseLiabilities	8700000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Reserves DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsReserves	8800000	usd
CY2021Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Reserves DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsReserves	0	usd
CY2022Q4	fibk	Deferred Tax Assets Contract Incentives DeferredTaxAssetsContractIncentives	8500000	usd
CY2021Q4	fibk	Deferred Tax Assets Contract Incentives DeferredTaxAssetsContractIncentives	0	usd
CY2022Q4	fibk	Deferred Tax Assets Discount On Acquired Investment Securities DeferredTaxAssetsDiscountOnAcquiredInvestmentSecurities	13400000	usd
CY2021Q4	fibk	Deferred Tax Assets Discount On Acquired Investment Securities DeferredTaxAssetsDiscountOnAcquiredInvestmentSecurities	0	usd
CY2022Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	2400000	usd
CY2021Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	3500000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	323200000	usd
CY2021Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	71100000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Property Plant And Equipment DeferredTaxLiabilitiesPropertyPlantAndEquipment	17300000	usd
CY2021Q4	us-gaap	Deferred Tax Liabilities Property Plant And Equipment DeferredTaxLiabilitiesPropertyPlantAndEquipment	7700000	usd
CY2022Q4	fibk	Deferred Tax Liabilities Deferred Loan Costs DeferredTaxLiabilitiesDeferredLoanCosts	3300000	usd
CY2021Q4	fibk	Deferred Tax Liabilities Deferred Loan Costs DeferredTaxLiabilitiesDeferredLoanCosts	1900000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Derivatives DeferredTaxLiabilitiesDerivatives	0	usd
CY2021Q4	us-gaap	Deferred Tax Liabilities Derivatives DeferredTaxLiabilitiesDerivatives	1000000.0	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Investment In Noncontrolled Affiliates DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates	1300000	usd
CY2021Q4	us-gaap	Deferred Tax Liabilities Investment In Noncontrolled Affiliates DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates	900000	usd
CY2022Q4	fibk	Deferred Tax Assets Rightof Use Asset DeferredTaxAssetsRightofUseAsset	11300000	usd
CY2021Q4	fibk	Deferred Tax Assets Rightof Use Asset DeferredTaxAssetsRightofUseAsset	8300000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Prepaid Expenses DeferredTaxLiabilitiesPrepaidExpenses	900000	usd
CY2021Q4	us-gaap	Deferred Tax Liabilities Prepaid Expenses DeferredTaxLiabilitiesPrepaidExpenses	700000	usd
CY2022Q4	fibk	Deferred Tax Liabilities Agency Stock Dividends DeferredTaxLiabilitiesAgencyStockDividends	1200000	usd
CY2021Q4	fibk	Deferred Tax Liabilities Agency Stock Dividends DeferredTaxLiabilitiesAgencyStockDividends	1200000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Goodwill And Intangible Assets DeferredTaxLiabilitiesGoodwillAndIntangibleAssets	68200000	usd
CY2021Q4	us-gaap	Deferred Tax Liabilities Goodwill And Intangible Assets DeferredTaxLiabilitiesGoodwillAndIntangibleAssets	51200000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Mortgage Servicing Rights DeferredTaxLiabilitiesMortgageServicingRights	7400000	usd
CY2021Q4	us-gaap	Deferred Tax Liabilities Mortgage Servicing Rights DeferredTaxLiabilitiesMortgageServicingRights	6800000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Other DeferredTaxLiabilitiesOther	1800000	usd
CY2021Q4	us-gaap	Deferred Tax Liabilities Other DeferredTaxLiabilitiesOther	700000	usd
CY2022Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	112700000	usd
CY2021Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	80400000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	210500000	usd
CY2021Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	9300000	usd
CY2022Q4	us-gaap	Tax Credit Carryforward Amount TaxCreditCarryforwardAmount	2700000	usd
CY2022Q4	us-gaap	Income Taxes Receivable IncomeTaxesReceivable	38300000	usd
CY2021Q4	us-gaap	Income Taxes Receivable IncomeTaxesReceivable	16100000	usd
CY2022	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Expiration Period SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod	P10Y
CY2021Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	23252	shares
CY2021Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	14.37
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod	22684	shares
CY2022	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	14.37
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod	568	shares
CY2022	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Forfeitures In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice	14.37
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	0	shares
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	0
CY2022	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P0Y
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	0	shares
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice	0
CY2022	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Weighted Average Remaining Contractual Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1	P0Y
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Vested And Expected To Vest Outstanding Aggregate Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue	0	usd
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Vested And Expected To Vest Outstanding Aggregate Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue	0	usd
CY2022	us-gaap	Employee Service Share Based Compensation Tax Benefit From Exercise Of Stock Options EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions	100000	usd
CY2021	us-gaap	Employee Service Share Based Compensation Tax Benefit From Exercise Of Stock Options EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions	200000	usd
CY2020	us-gaap	Employee Service Share Based Compensation Tax Benefit From Exercise Of Stock Options EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions	500000	usd
CY2022	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	-613100000	usd
CY2022	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	-152400000	usd
CY2022	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	-460700000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	-24400000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax	-5500000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	-18900000	usd
CY2022	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	200000	usd
CY2022	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesTax	100000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Transfers From Held To Maturity To Available For Sale Securities Net Of Tax OtherComprehensiveIncomeLossTransfersFromHeldToMaturityToAvailableForSaleSecuritiesNetOfTax	-100000	usd
CY2022	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Before Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossBeforeTax	26100000	usd
CY2022	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossTax	6600000	usd
CY2022	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss After Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossAfterTax	19500000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Before Reclassification Adjustments Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesBeforeReclassificationAdjustmentsTax	-6700000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationTax	1800000	usd
CY2022	us-gaap	Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet	-4900000	usd
CY2022Q1	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-621300000	usd
CY2022Q1	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-155200000	usd
CY2022Q1	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-466100000	usd
CY2021	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	-113700000	usd
CY2021	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	-28700000	usd
CY2021	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	-85000000.0	usd
CY2021	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	1100000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax	300000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	800000	usd
CY2021	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Before Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossBeforeTax	-20200000	usd
CY2021	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossTax	-5100000	usd
CY2021	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss After Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossAfterTax	-15100000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Before Reclassification Adjustments Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesBeforeReclassificationAdjustmentsTax	4200000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationTax	-1100000	usd
CY2021	us-gaap	Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet	3100000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-90400000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-22800000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-67600000	usd
CY2020	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	61800000	usd
CY2020	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	15800000	usd
CY2020	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	46000000.0	usd
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	300000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax	100000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	200000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Available For Sale Securities Before Reclassification Adjustments Tax OtherComprehensiveIncomeLossAvailableForSaleSecuritiesBeforeReclassificationAdjustmentsTax	200000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationTax	0	usd
CY2022Q4	us-gaap	Long Term Debt LongTermDebt	98900000	usd
CY2021Q4	us-gaap	Long Term Debt LongTermDebt	98700000	usd
CY2020	us-gaap	Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet	200000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Net Unamortized Gain Loss Arising During Period Before Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax	-500000	usd
CY2020	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodTax	-100000	usd
CY2020	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Net Of Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax	-400000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	61200000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	15600000	usd
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	45600000	usd
CY2022	us-gaap	Held To Maturity Securities Transferred Security Unrealized Gain Loss HeldToMaturitySecuritiesTransferredSecurityUnrealizedGainLoss	-4500000	usd
CY2021	us-gaap	Held To Maturity Securities Transferred Security Unrealized Gain Loss HeldToMaturitySecuritiesTransferredSecurityUnrealizedGainLoss	15000000.0	usd
CY2022	fibk	Net Unrealized Gain Loss On Derivatives Net Of Tax NetUnrealizedGainLossOnDerivativesNetOfTax	-1600000	usd
CY2021	fibk	Net Unrealized Gain Loss On Derivatives Net Of Tax NetUnrealizedGainLossOnDerivativesNetOfTax	3000000.0	usd
CY2022	us-gaap	Loans And Leases Receivable Related Parties Period Increase Decrease LoansAndLeasesReceivableRelatedPartiesPeriodIncreaseDecrease	0	usd
CY2021	us-gaap	Loans And Leases Receivable Related Parties Proceeds LoansAndLeasesReceivableRelatedPartiesProceeds	700000	usd

Files In Submission

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0000860413-23-000045.txt	Edgar Link	pending
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fibk-20221231_cal.xml	Edgar Link	unprocessable

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$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

2023 Q1 Form 10-K Financial Statement

#000086041323000045 Filed on February 24, 2023

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$FIBK FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

2023 Q1 Form 10-K Financial Statement

#000086041323000045 Filed on February 24, 2023

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk