$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

$fibk Overview Financials Valuation SEC Filings Company Website

2021 Q4 Form 10-K Financial Statement

#000086041322000065 Filed on February 25, 2022

Income Statement

Concept	2021 Q4	2021	2020 Q4
Revenue		$505.5M
YoY Change		-3.46%
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	$11.00M	$44.40M	$12.10M
YoY Change	-9.09%	-1.55%	24.74%
% of Gross Profit
Operating Expenses
YoY Change
Operating Profit
YoY Change
Interest Expense		$17.30M
YoY Change		-34.96%
% of Operating Profit
Other Income/Expense, Net
YoY Change
Pretax Income	$66.50M	$247.8M	$61.70M
YoY Change	7.78%	18.39%	-10.45%
Income Tax	$15.40M	$55.70M	$14.80M
% Of Pretax Income	23.16%	22.48%	23.99%
Net Earnings	$51.10M	$192.1M	$46.90M
YoY Change	8.96%	19.17%	-10.5%
Net Earnings / Revenue		38.0%
Basic Earnings Per Share		$3.12
Diluted Earnings Per Share	$826.9K	$3.11	$756.5K
COMMON SHARES
Basic Shares Outstanding		61.65M shares
Diluted Shares Outstanding		61.74M shares

Balance Sheet

Concept	2021 Q4	2021	2020 Q4
SHORT-TERM ASSETS
Cash & Short-Term Investments	$8.853B		$6.337B
YoY Change	39.7%		53.47%
Cash & Equivalents	$2.345B		$2.277B
Short-Term Investments	$6.508B		$4.060B
Other Short-Term Assets
YoY Change
Inventory
Prepaid Expenses
Receivables
Other Receivables
Total Short-Term Assets
YoY Change
LONG-TERM ASSETS
Property, Plant & Equipment	$299.6M	$300.0M	$312.3M
YoY Change	-4.07%	-3.85%	2.06%
Goodwill	$621.6M		$621.6M
YoY Change	0.0%		0.0%
Intangibles
YoY Change
Long-Term Investments
YoY Change
Other Assets
YoY Change
Total Long-Term Assets
YoY Change
TOTAL ASSETS
Total Short-Term Assets
Total Long-Term Assets
Total Assets	$19.67B	$19.67B	$17.65B
YoY Change	11.46%	11.46%	20.52%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable	$4.000M	$4.000M	$6.000M
YoY Change	-33.33%	-33.33%	-50.0%
Accrued Expenses
YoY Change
Deferred Revenue
YoY Change
Short-Term Debt	$1.051B	$1.051B	$1.091B
YoY Change	-3.67%	-3.67%	56.3%
Long-Term Debt Due
YoY Change
Total Short-Term Liabilities	$148.0M	$148.0M	$144.0M
YoY Change	2.78%	2.78%	10.77%
LONG-TERM LIABILITIES
Long-Term Debt	$199.0M	$199.0M	$199.0M
YoY Change	0.0%	0.0%	97.03%
Other Long-Term Liabilities	$4.000M	$4.000M	$4.000M
YoY Change	0.0%	0.0%
Total Long-Term Liabilities	$203.0M	$203.0M	$203.0M
YoY Change	0.0%	0.0%	100.99%
TOTAL LIABILITIES
Total Short-Term Liabilities	$148.0M	$148.0M	$144.0M
Total Long-Term Liabilities	$203.0M	$203.0M	$203.0M
Total Liabilities	$17.69B	$17.69B	$15.69B
YoY Change	12.72%	12.72%	24.22%
SHAREHOLDERS EQUITY
Retained Earnings	$1.053B		$962.1M
YoY Change	9.41%		0.89%
Common Stock	$0.00		$0.00
YoY Change
Preferred Stock
YoY Change
Treasury Stock (at cost)
YoY Change
Treasury Stock Shares
Shareholders Equity	$1.987B	$1.987B	$1.960B
YoY Change
Total Liabilities & Shareholders Equity	$19.67B	$19.67B	$17.65B
YoY Change	11.46%	11.46%	20.52%

Cashflow Statement

Concept	2021 Q4	2021	2020 Q4
OPERATING ACTIVITIES
Net Income	$51.10M	$192.1M	$46.90M
YoY Change	8.96%	19.17%	-10.5%
Depreciation, Depletion And Amortization	$11.00M	$44.40M	$12.10M
YoY Change	-9.09%	-1.55%	24.74%
Cash From Operating Activities	$76.90M	$282.3M	$105.5M
YoY Change	-27.11%	5.22%	49.43%
INVESTING ACTIVITIES
Capital Expenditures	-$4.800M	-$10.30M	-$12.50M
YoY Change	-61.6%	-65.89%	95.31%
Acquisitions		$0.00
YoY Change
Other Investing Activities	-$241.3M	-$2.110B	-$221.2M
YoY Change	9.09%	14.68%	36.97%
Cash From Investing Activities	-$246.1M	-$2.120B	-$233.7M
YoY Change	5.31%	13.38%	39.19%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net		$5.000M
YoY Change		-95.68%
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	280.5M	$1.906B	544.4M
YoY Change	-48.48%	-31.98%	-670.05%
NET CHANGE
Cash From Operating Activities	76.90M	$282.3M	105.5M
Cash From Investing Activities	-246.1M	-$2.120B	-233.7M
Cash From Financing Activities	280.5M	$1.906B	544.4M
Net Change In Cash	111.3M	$68.00M	416.2M
YoY Change	-73.26%	-94.33%	-315.87%
FREE CASH FLOW
Cash From Operating Activities	$76.90M	$282.3M	$105.5M
Capital Expenditures	-$4.800M	-$10.30M	-$12.50M
Free Cash Flow	$81.70M	$292.6M	$118.0M
YoY Change	-30.76%	-1.98%	53.25%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2021	dei	Entity Central Index Key EntityCentralIndexKey	0000860413
CY2021	dei	Document Period End Date DocumentPeriodEndDate	2021-12-31
CY2021	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2021	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2021
CY2021	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
CY2021	dei	Amendment Flag AmendmentFlag	false
CY2019	us-gaap	Accounting Standards Update Extensible List AccountingStandardsUpdateExtensibleList	http://fasb.org/us-gaap/2021-01-31#AccountingStandardsUpdate201613Member
CY2021	dei	Document Type DocumentType	10-K
CY2021	dei	Document Annual Report DocumentAnnualReport	true
CY2021	dei	Document Transition Report DocumentTransitionReport	false
CY2021	dei	Entity File Number EntityFileNumber	001-34653
CY2021	dei	Entity Registrant Name EntityRegistrantName	FIRST INTERSTATE BANCSYSTEM, INC.
CY2021	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	MT
CY2021	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	81-0331430
CY2021	dei	Entity Address Address Line1 EntityAddressAddressLine1	401 North 31st Street
CY2021	dei	Entity Address City Or Town EntityAddressCityOrTown	Billings,
CY2021	dei	Entity Address State Or Province EntityAddressStateOrProvince	MT
CY2021	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2021	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	59101
CY2021	dei	City Area Code CityAreaCode	406
CY2021	dei	Local Phone Number LocalPhoneNumber	255-5311
CY2021	dei	Security12b Title Security12bTitle	Class A common stock, no par value
CY2021	dei	Trading Symbol TradingSymbol	FIBK
CY2021	dei	Security Exchange Name SecurityExchangeName	NASDAQ
CY2021	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	Yes
CY2021	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
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CY2019	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	11200000	USD
CY2021	us-gaap	Other Noninterest Expense OtherNoninterestExpense	65700000	USD
CY2020	us-gaap	Other Noninterest Expense OtherNoninterestExpense	60400000	USD
CY2019	us-gaap	Other Noninterest Expense OtherNoninterestExpense	63600000	USD
CY2021	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	11600000	USD
CY2020	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	0	USD
CY2019	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	20300000	USD
CY2020	us-gaap	Noninterest Expense NoninterestExpense	387500000	USD
CY2019	us-gaap	Noninterest Expense NoninterestExpense	388600000	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	209300000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	235100000	USD
CY2020	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	48100000	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	54100000	USD
CY2020	us-gaap	Net Income Loss NetIncomeLoss	161200000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	181000000.0	USD
CY2020	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.53
CY2019	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.84
CY2021	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	3.11
CY2020	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.53
CY2019	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.83
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	USD
CY2020	us-gaap	Net Income Loss NetIncomeLoss	161200000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	181000000.0	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
CY2019	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	54900000	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
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	100000	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
CY2019	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	-6000000.0	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
CY2019	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
CY2021	us-gaap	Unrealized Gain Loss On Derivatives UnrealizedGainLossOnDerivatives	4200000	USD
CY2020	us-gaap	Defined Benefit Plan Actuarial Gain Loss DefinedBenefitPlanActuarialGainLoss	-500000	USD
CY2020	us-gaap	Unrealized Gain Loss On Derivatives UnrealizedGainLossOnDerivatives	200000	USD
CY2019	us-gaap	Unrealized Gain Loss On Derivatives UnrealizedGainLossOnDerivatives	0	USD
CY2021	us-gaap	Defined Benefit Plan Actuarial Gain Loss DefinedBenefitPlanActuarialGainLoss	0	USD
CY2019	us-gaap	Defined Benefit Plan Actuarial Gain Loss DefinedBenefitPlanActuarialGainLoss	-800000	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
CY2019	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	48000000.0	USD
CY2021	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-22800000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	15600000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	12400000	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
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	35600000	USD
CY2021	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	124500000	USD
CY2020	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	206800000	USD
CY2019	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	216600000	USD
CY2018Q4	us-gaap	Stockholders Equity StockholdersEquity	1693900000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	181000000.0	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	35600000	USD
CY2019	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	43560	shares
CY2019	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	2500000	USD
CY2019	us-gaap	Stock Issued During Period Shares New Issues StockIssuedDuringPeriodSharesNewIssues	4356973	shares
CY2019	us-gaap	Stock Issued During Period Value New Issues StockIssuedDuringPeriodValueNewIssues	176100000	USD
CY2019	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	212587	shares
CY2019Q4	us-gaap	Common Stock Value CommonStockValue	0	USD
CY2019	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Nonvested Options Forfeited Number Of Shares SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedNumberOfShares	46198	shares
CY2019	us-gaap	Stock Issued During Period Value Restricted Stock Award Forfeitures StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0	USD
CY2019	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	143222	shares
CY2019	fibk	Shares Tendered In Payment Of Option Price And Income Tax Withholding Amounts Shares SharesTenderedInPaymentOfOptionPriceAndIncomeTaxWithholdingAmountsShares	47971	shares
CY2019	us-gaap	Stock Issued During Period Value Stock Options Exercised Net Of Tax Benefit Expense StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	1000000.0	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	8000000.0	USD
CY2019	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.24
CY2019	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Stock Options Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	-79200000	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
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	USD
CY2020	us-gaap	Net Income Loss NetIncomeLoss	161200000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	181000000.0	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
CY2019	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	13900000	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
CY2019	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	1500000	USD
CY2021	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	44400000	USD
CY2020	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	45100000	USD
CY2019	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	38700000	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
CY2019	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	-8900000	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
CY2019	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	100000	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
CY2019	us-gaap	Gain Loss On Sale Of Loans And Leases GainLossOnSaleOfLoansAndLeases	30500000	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
CY2019	us-gaap	Gain Loss On Sale Of Other Investments GainLossOnSaleOfOtherInvestments	0	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
CY2019	us-gaap	Gains Losses On Sales Of Other Real Estate GainsLossesOnSalesOfOtherRealEstate	3600000	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
CY2019	fibk	Impairmentof Real Estateand Other Assets Pending Disposal ImpairmentofRealEstateandOtherAssetsPendingDisposal	900000	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
CY2019	us-gaap	Mortgage Servicing Rights Msr Impairment Recovery MortgageServicingRightsMSRImpairmentRecovery	400000	USD
CY2021	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	5000000.0	USD
CY2020	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-6600000	USD
CY2019	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	5400000	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
CY2019	us-gaap	Life Insurance Corporate Or Bank Owned Change In Value LifeInsuranceCorporateOrBankOwnedChangeInValue	6700000	USD
CY2021	us-gaap	Share Based Compensation ShareBasedCompensation	8900000	USD
CY2020	us-gaap	Share Based Compensation ShareBasedCompensation	7500000	USD
CY2019	us-gaap	Share Based Compensation ShareBasedCompensation	8000000.0	USD
CY2021	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	817400000	USD
CY2019	us-gaap	Increase Decrease In Accounts Payable And Accrued Liabilities IncreaseDecreaseInAccountsPayableAndAccruedLiabilities	-7800000	USD
CY2021	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	282300000	USD
CY2020	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	1404200000	USD
CY2019	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	1015600000	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
CY2019	us-gaap	Proceeds From Sale Of Mortgage Loans Held For Sale ProceedsFromSaleOfMortgageLoansHeldForSale	971200000	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
CY2019	us-gaap	Increase Decrease In Accrued Interest Receivable Net IncreaseDecreaseInAccruedInterestReceivableNet	-300000	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
CY2019	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	22100000	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
CY2019	us-gaap	Increase Decrease In Interest Payable Net IncreaseDecreaseInInterestPayableNet	-13500000	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
CY2020	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	268300000	USD
CY2020	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	0	USD
CY2019	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	127300000	USD
CY2021	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	1238000000	USD
CY2019	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	0	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
CY2019	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	1270000000	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
CY2019	us-gaap	Proceeds From Sale And Maturity Of Held To Maturity Securities ProceedsFromSaleAndMaturityOfHeldToMaturitySecurities	35600000	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
CY2019	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	978600000	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
CY2019	us-gaap	Proceeds From Life Insurance Policies ProceedsFromLifeInsurancePolicies	3200000	USD
CY2021	us-gaap	Payments For Proceeds From Other Loans And Leases PaymentsForProceedsFromOtherLoansAndLeases	-458500000	USD
CY2020	us-gaap	Payments For Proceeds From Other Loans And Leases PaymentsForProceedsFromOtherLoansAndLeases	901300000	USD
CY2019	us-gaap	Payments For Proceeds From Other Loans And Leases PaymentsForProceedsFromOtherLoansAndLeases	81400000	USD
CY2021	fibk	Payments For Proceeds From Recoveriesof Loans Previously Chargedoff PaymentsForProceedsFromRecoveriesofLoansPreviouslyChargedoff	-9300000	USD
CY2020	fibk	Payments For Proceeds From Recoveriesof Loans Previously Chargedoff PaymentsForProceedsFromRecoveriesofLoansPreviouslyChargedoff	-6700000	USD
CY2019	fibk	Payments For Proceeds From Recoveriesof Loans Previously Chargedoff PaymentsForProceedsFromRecoveriesofLoansPreviouslyChargedoff	-9700000	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
CY2019	us-gaap	Proceeds From Sale Of Wholly Owned Real Estate And Real Estate Acquired In Settlement Of Loans ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans	25400000	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
CY2019	fibk	Proceeds From The Sale Of Health Savings Accounts ProceedsFromTheSaleOfHealthSavingsAccounts	300000	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
CY2019	us-gaap	Proceeds From Sale Of Other Investments ProceedsFromSaleOfOtherInvestments	0	USD
CY2021	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	0	USD
CY2020	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	0	USD
CY2019	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	298400000	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
CY2019	us-gaap	Payments For Proceeds From Productive Assets PaymentsForProceedsFromProductiveAssets	16600000	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
CY2019	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-16800000	USD
CY2021	us-gaap	Increase Decrease In Deposits IncreaseDecreaseInDeposits	2052600000	USD
CY2020	us-gaap	Increase Decrease In Deposits IncreaseDecreaseInDeposits	2553500000	USD
CY2019	us-gaap	Increase Decrease In Deposits IncreaseDecreaseInDeposits	276200000	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
CY2019	us-gaap	Proceeds From Payments For In Securities Sold Under Agreements To Repurchase ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase	-45200000	USD
CY2021	us-gaap	Repayments Of Other Short Term Debt RepaymentsOfOtherShortTermDebt	0	USD
CY2020	us-gaap	Repayments Of Other Short Term Debt RepaymentsOfOtherShortTermDebt	0	USD
CY2019	us-gaap	Repayments Of Other Short Term Debt RepaymentsOfOtherShortTermDebt	4100000	USD
CY2021	us-gaap	Proceeds From Repayments Of Other Long Term Debt ProceedsFromRepaymentsOfOtherLongTermDebt	0	USD
CY2020	us-gaap	Proceeds From Repayments Of Other Long Term Debt ProceedsFromRepaymentsOfOtherLongTermDebt	-100000	USD
CY2019	us-gaap	Proceeds From Repayments Of Other Long Term Debt ProceedsFromRepaymentsOfOtherLongTermDebt	-2000000.0	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
CY2019	us-gaap	Proceeds From Issuance Of Unsecured Debt ProceedsFromIssuanceOfUnsecuredDebt	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
CY2019	us-gaap	Proceeds From Issuance Of Common Stock ProceedsFromIssuanceOfCommonStock	1000000.0	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
CY2019	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	2500000	USD
CY2021	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	101600000	USD
CY2020	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	128600000	USD
CY2019	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	79200000	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
CY2019	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	144300000	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
CY2019	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	254800000	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
CY2018Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	822000000.0	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
CY2021	us-gaap	Income Taxes Paid IncomeTaxesPaid	56800000	USD
CY2020	us-gaap	Income Taxes Paid IncomeTaxesPaid	54400000	USD
CY2019	us-gaap	Income Taxes Paid IncomeTaxesPaid	51200000	USD
CY2021	us-gaap	Interest Paid Net InterestPaidNet	19300000	USD
CY2020	us-gaap	Interest Paid Net InterestPaidNet	32900000	USD
CY2019	us-gaap	Interest Paid Net InterestPaidNet	54700000	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
CY2019	us-gaap	Proceeds From Sale Of Held To Maturity Securities ProceedsFromSaleOfHeldToMaturitySecurities	281100000	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
CY2019	fibk	Non Cash Transfer Of Securities From Available For Sale To Held To Maturity NonCashTransferOfSecuritiesFromAvailableForSaleToHeldToMaturity	0	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
CY2019	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	39600000	USD
CY2021	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	900000	USD
CY2020	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	3300000	USD
CY2019	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	14100000	USD
CY2021	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	3600000	USD
CY2020	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	11700000	USD
CY2019	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	7300000	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
CY2019	us-gaap	Noncash Or Part Noncash Acquisition Investments Acquired1 NoncashOrPartNoncashAcquisitionInvestmentsAcquired1	78700000	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
CY2019	fibk	Noncash Or Part Noncash Acquisition Loans Held For Sale Acquired NoncashOrPartNoncashAcquisitionLoansHeldForSaleAcquired	500000	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
CY2019	fibk	Noncash Or Part Noncash Acquisition Loans Acquired NoncashOrPartNoncashAcquisitionLoansAcquired	416600000	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
CY2019	us-gaap	Noncash Or Part Noncash Acquisition Fixed Assets Acquired1 NoncashOrPartNoncashAcquisitionFixedAssetsAcquired1	24600000	USD
CY2021	fibk	Noncash Or Part Noncash Acquisition Goodwill NoncashOrPartNoncashAcquisitionGoodwill	0	USD
CY2020	fibk	Noncash Or Part Noncash Acquisition Goodwill NoncashOrPartNoncashAcquisitionGoodwill	0	USD
CY2019	fibk	Noncash Or Part Noncash Acquisition Goodwill NoncashOrPartNoncashAcquisitionGoodwill	75300000	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
CY2019	us-gaap	Noncash Or Part Noncash Acquisition Intangible Assets Acquired1 NoncashOrPartNoncashAcquisitionIntangibleAssetsAcquired1	16600000	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
CY2019	fibk	Noncash Or Part Noncash Acquisition Company Owned Life Insurance Acquired NoncashOrPartNoncashAcquisitionCompanyOwnedLifeInsuranceAcquired	15200000	USD
CY2021	fibk	Noncash Or Part Noncash Acquisition Interest Receivable Acquired NoncashOrPartNoncashAcquisitionInterestReceivableAcquired	0	USD
CY2020	fibk	Noncash Or Part Noncash Acquisition Interest Receivable Acquired NoncashOrPartNoncashAcquisitionInterestReceivableAcquired	0	USD
CY2019	fibk	Noncash Or Part Noncash Acquisition Interest Receivable Acquired NoncashOrPartNoncashAcquisitionInterestReceivableAcquired	2200000	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
CY2019	fibk	Noncash Or Part Noncash Acquisition Other Real Estate Owned Acquired NoncashOrPartNoncashAcquisitionOtherRealEstateOwnedAcquired	2400000	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
CY2019	us-gaap	Noncash Or Part Noncash Acquisition Other Assets Acquired1 NoncashOrPartNoncashAcquisitionOtherAssetsAcquired1	6500000	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
CY2019	us-gaap	Noncash Or Part Noncash Acquisition Value Of Assets Acquired1 NoncashOrPartNoncashAcquisitionValueOfAssetsAcquired1	638600000	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
CY2019	fibk	Noncash Or Part Noncash Acquisition Deposits Acquired NoncashOrPartNoncashAcquisitionDepositsAcquired	706700000	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
CY2019	fibk	Noncash Or Part Noncash Acquisition Securities Sold Under Repurchase Agreements NoncashOrPartNoncashAcquisitionSecuritiesSoldUnderRepurchaseAgreements	30400000	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
CY2019	us-gaap	Noncash Or Part Noncash Acquisition Payables Assumed1 NoncashOrPartNoncashAcquisitionPayablesAssumed1	19900000	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
CY2019	us-gaap	Noncash Or Part Noncash Acquisition Debt Assumed1 NoncashOrPartNoncashAcquisitionDebtAssumed1	4100000	USD
CY2021	fibk	Noncashor Part Noncash Acquisition Deferred Tax Liability NoncashorPartNoncashAcquisitionDeferredTaxLiability	0	USD
CY2020	fibk	Noncashor Part Noncash Acquisition Deferred Tax Liability NoncashorPartNoncashAcquisitionDeferredTaxLiability	0	USD
CY2019	fibk	Noncashor Part Noncash Acquisition Deferred Tax Liability NoncashorPartNoncashAcquisitionDeferredTaxLiability	100000	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
CY2019	us-gaap	Noncash Or Part Noncash Acquisition Value Of Liabilities Assumed1 NoncashOrPartNoncashAcquisitionValueOfLiabilitiesAssumed1	761200000	USD
CY2021Q4	fibk	Numberof Subsidiary Banks NumberofSubsidiaryBanks	1	subsidiary
CY2021	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:120%">Use of Estimates.</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%"> 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 fair valuations of investment securities and other financial instruments.</span>
CY2021Q4	fibk	Federal Reserve Bank Unrestricted Cash Held Carrying Value FederalReserveBankUnrestrictedCashHeldCarryingValue	2166100000	USD
CY2020Q4	fibk	Federal Reserve Bank Unrestricted Cash Held Carrying Value FederalReserveBankUnrestrictedCashHeldCarryingValue	1989300000	USD
CY2021Q4	us-gaap	Compensating Balance Amount CompensatingBalanceAmount	0	USD
CY2021Q4	us-gaap	Accrued Investment Income Receivable AccruedInvestmentIncomeReceivable	16600000	USD
CY2020Q4	us-gaap	Accrued Investment Income Receivable AccruedInvestmentIncomeReceivable	12600000	USD
CY2021Q4	us-gaap	Deferred Compensation Plan Assets DeferredCompensationPlanAssets	21400000	USD
CY2020Q4	us-gaap	Deferred Compensation Plan Assets DeferredCompensationPlanAssets	19100000	USD
CY2021Q4	us-gaap	Deferred Compensation Liability Current And Noncurrent DeferredCompensationLiabilityCurrentAndNoncurrent	21400000	USD
CY2020Q4	us-gaap	Deferred Compensation Liability Current And Noncurrent DeferredCompensationLiabilityCurrentAndNoncurrent	19100000	USD
CY2019	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
CY2021Q4	us-gaap	Federal Reserve Bank Stock FederalReserveBankStock	42800000	USD
CY2021Q4	us-gaap	Federal Home Loan Bank Stock FederalHomeLoanBankStock	10600000	USD
CY2021	us-gaap	Derivatives Methods Of Accounting Hedging Derivatives DerivativesMethodsOfAccountingHedgingDerivatives	or 2019.<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:115%">Derivatives and Hedging Activities. </span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:115%">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:8pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:115%">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 swap agreements are derivative instruments and convert a portion of the Company’s forecasted variable rate debt to a fixed rate (i.e., cash flow hedge) over the payment term of the interest rate swap. 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:8pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:115%">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 dedesignated 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:8pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">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:8pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">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:120%">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><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:120%">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. </span></div>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 o
CY2021	us-gaap	Hosting Arrangement Service Contract Implementation Cost Expense Amortization HostingArrangementServiceContractImplementationCostExpenseAmortization	4600000	USD
CY2020	us-gaap	Hosting Arrangement Service Contract Implementation Cost Expense Amortization HostingArrangementServiceContractImplementationCostExpenseAmortization	6000000	USD
CY2019	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
CY2021	us-gaap	Number Of Operating Segments NumberOfOperatingSegments	1	segment
CY2021	us-gaap	Advertising Expense AdvertisingExpense	2600000	USD
CY2020	us-gaap	Advertising Expense AdvertisingExpense	2800000	USD
CY2019	us-gaap	Advertising Expense AdvertisingExpense	4200000	USD
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Share Based Compensation Excess Tax Benefit Amount EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitAmount	-1200000	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
CY2021	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	9900000	USD
CY2020	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	10900000	USD
CY2019	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	11200000	USD
CY2021	us-gaap	Finite Lived Intangible Asset Useful Life FiniteLivedIntangibleAssetUsefulLife	P10Y
CY2020Q4	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Loss HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss	0	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	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
CY2020Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	3932700000	USD
CY2020Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	81700000	USD
CY2020Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	5700000	USD
CY2020Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	4008700000	USD
CY2020Q4	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	51600000	USD
CY2020Q4	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Gain HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain	3400000	USD
CY2021	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	2100000	USD
CY2020Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	55000000.0	USD
CY2021	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	3200000	USD
CY2020	us-gaap	Realized Investment Gains Losses RealizedInvestmentGainsLosses	0	USD
CY2019	us-gaap	Realized Investment Gains Losses RealizedInvestmentGainsLosses	0	USD
CY2021Q2	us-gaap	Debt Securities Available For Sale And Held To Maturity Amortized Cost Before Allowance For Credit Loss DebtSecuritiesAvailableForSaleAndHeldToMaturityAmortizedCostBeforeAllowanceForCreditLoss	646700000	USD
CY2021Q2	us-gaap	Debt Securities Available For Sale Transfer To Held To Maturity Allowance For Credit Loss Expense DebtSecuritiesAvailableForSaleTransferToHeldToMaturityAllowanceForCreditLossExpense	672200000	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
CY2020Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months	838600000	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
CY2020Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months Accumulated Loss DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	5500000	USD
CY2020Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger	16600000	USD
CY2020Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer Accumulated Loss DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	200000	USD
CY2020Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position DebtSecuritiesAvailableForSaleUnrealizedLossPosition	855200000	USD
CY2020Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Accumulated Loss DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss	5700000	USD
CY2021Q4	fibk	Available For Sale And Held To Maturity Securities In Unrealized Loss Positions Qualitative Disclosure Number Of Positions AvailableForSaleAndHeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions	285	Investments
CY2020Q4	fibk	Available For Sale And Held To Maturity Securities In Unrealized Loss Positions Qualitative Disclosure Number Of Positions AvailableForSaleAndHeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions	181	Investments
CY2021Q4	us-gaap	Impaired Financing Receivable Related Allowance ImpairedFinancingReceivableRelatedAllowance	0	USD
CY2019	us-gaap	Impairment Of Investments ImpairmentOfInvestments	0	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities Next Rolling Twelve Months Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis	647400000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities Next Rolling Twelve Months Fair Value AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue	742800000	USD
CY2021Q4	us-gaap	Held To Maturity Securities Debt Maturities Within One Year Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount	696400000	USD
CY2021Q4	us-gaap	Held To Maturity Securities Debt Maturities Next Rolling Twelve Months Fair Value HeldToMaturitySecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue	687200000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Two Through Five Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis	1482400000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Two Through Five Fair Value AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue	1577400000	USD
CY2021Q4	us-gaap	Held To Maturity Securities Debt Maturities After One Through Five Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount	374500000	USD
CY2021Q4	us-gaap	Held To Maturity Securities Debt Maturities Rolling Year Two Through Five Fair Value HeldToMaturitySecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue	363600000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Six Through Ten Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis	1313400000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Six Through Ten Fair Value AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue	1165600000	USD
CY2021Q4	us-gaap	Held To Maturity Securities Debt Maturities After Five Through Ten Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount	275600000	USD
CY2021Q4	us-gaap	Held To Maturity Securities Debt Maturities Rolling Year Six Through Ten Fair Value HeldToMaturitySecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue	276200000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling After Year Ten Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenAmortizedCostBasis	1416100000	USD
CY2021Q4	us-gaap	Available For Sale Securities Debt Maturities Rolling After Year Ten Fair Value AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenFairValue	1334700000	USD
CY2021Q4	us-gaap	Held To Maturity Securities Debt Maturities After Ten Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount	341100000	USD
CY2021Q4	us-gaap	Held To Maturity Securities Debt Maturities Rolling After Ten Years Fair Value HeldToMaturitySecuritiesDebtMaturitiesRollingAfterTenYearsFairValue	340500000	USD
CY2021Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	4859300000	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	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	1667500000	USD
CY2021Q4	fibk	Pledged Assets Separately Reported Securities Pledged For Repurchase Agreements At Cost PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtCost	2617800000	USD
CY2020Q4	fibk	Pledged Assets Separately Reported Securities Pledged For Repurchase Agreements At Cost PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtCost	2323000000	USD
CY2021Q4	us-gaap	Pledged Assets Separately Reported Securities Pledged For Repurchase Agreements At Fair Value PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtFairValue	2610800000	USD
CY2020Q4	us-gaap	Pledged Assets Separately Reported Securities Pledged For Repurchase Agreements At Fair Value PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtFairValue	2383600000	USD
CY2021Q4	us-gaap	Loans Receivable Held For Sale Net Not Part Of Disposal Group LoansReceivableHeldForSaleNetNotPartOfDisposalGroup	30100000	USD
CY2020Q4	us-gaap	Loans Receivable Held For Sale Net Not Part Of Disposal Group LoansReceivableHeldForSaleNetNotPartOfDisposalGroup	74000000	USD
CY2021Q4	us-gaap	Loans And Leases Receivable Deferred Income LoansAndLeasesReceivableDeferredIncome	12300000	USD
CY2020Q4	us-gaap	Loans And Leases Receivable Deferred Income LoansAndLeasesReceivableDeferredIncome	21000000.0	USD
CY2021Q4	us-gaap	Loans And Leases Receivable Gross Carrying Amount LoansAndLeasesReceivableGrossCarryingAmount	9331700000	USD
CY2020Q4	us-gaap	Loans And Leases Receivable Gross Carrying Amount LoansAndLeasesReceivableGrossCarryingAmount	9807500000	USD
CY2021Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	122300000	USD
CY2020Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	144300000	USD
CY2021Q4	us-gaap	Loans And Leases Receivable Net Reported Amount LoansAndLeasesReceivableNetReportedAmount	9209400000	USD
CY2020Q4	us-gaap	Loans And Leases Receivable Net Reported Amount LoansAndLeasesReceivableNetReportedAmount	9663200000	USD
CY2021Q3	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	144300000	USD
CY2021	us-gaap	Provision For Loan Lease And Other Losses ProvisionForLoanLeaseAndOtherLosses	-14700000	USD
CY2021	us-gaap	Financing Receivable Allowance For Credit Losses Write Offs FinancingReceivableAllowanceForCreditLossesWriteOffs	16600000	USD
CY2021	us-gaap	Financing Receivable Allowance For Credit Losses Recovery FinancingReceivableAllowanceForCreditLossesRecovery	9300000	USD
CY2021Q4	us-gaap	Financing Receivable Allowance For Credit Losses FinancingReceivableAllowanceForCreditLosses	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	Provision For Loan Lease And Other Losses ProvisionForLoanLeaseAndOtherLosses	55500000	USD
CY2020	us-gaap	Financing Receivable Allowance For Credit Losses Write Offs FinancingReceivableAllowanceForCreditLossesWriteOffs	20900000	USD
CY2020	us-gaap	Financing Receivable Allowance For Credit Losses Recovery FinancingReceivableAllowanceForCreditLossesRecovery	6700000	USD
CY2020Q4	us-gaap	Financing Receivable Allowance For Credit Losses FinancingReceivableAllowanceForCreditLosses	144300000	USD
CY2018Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	73000000.0	USD
CY2019	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	13900000	USD
CY2019	us-gaap	Allowance For Loan And Lease Losses Write Offs AllowanceForLoanAndLeaseLossesWriteOffs	23600000	USD
CY2019	us-gaap	Allowance For Loan And Lease Loss Recovery Of Bad Debts AllowanceForLoanAndLeaseLossRecoveryOfBadDebts	9700000	USD
CY2019Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	73000000.0	USD
CY2019Q4	us-gaap	Financing Receivable Allowance For Credit Losses Individually Evaluated For Impairment1 FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1	3600000	USD
CY2019Q4	us-gaap	Financing Receivable Allowance For Credit Losses Collectively Evaluated For Impairment FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment	69400000	USD
CY2019Q4	us-gaap	Loans And Leases Receivable Allowance LoansAndLeasesReceivableAllowance	73000000.0	USD
CY2019Q4	us-gaap	Financing Receivable Individually Evaluated For Impairment FinancingReceivableIndividuallyEvaluatedForImpairment	64700000	USD
CY2019Q4	us-gaap	Financing Receivable Collectively Evaluated For Impairment FinancingReceivableCollectivelyEvaluatedForImpairment	8871500000	USD
CY2019Q4	fibk	Loans And Leases Receivable Held For Investment Evaluated For Impairment LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment	8936200000	USD
CY2021Q4	us-gaap	Loans And Leases Receivable Collateral For Secured Borrowings LoansAndLeasesReceivableCollateralForSecuredBorrowings	11700000	USD
CY2020Q4	us-gaap	Loans And Leases Receivable Collateral For Secured Borrowings LoansAndLeasesReceivableCollateralForSecuredBorrowings	17500000	USD
CY2020	us-gaap	Impaired Financing Receivable Average Recorded Investment ImpairedFinancingReceivableAverageRecordedInvestment	64700000	USD
CY2020	us-gaap	Impaired Financing Receivable Interest Income Accrual Method ImpairedFinancingReceivableInterestIncomeAccrualMethod	300000	USD
CY2021Q4	us-gaap	Loans And Leases Receivable Impaired Nonperforming Accrual Of Interest LoansAndLeasesReceivableImpairedNonperformingAccrualOfInterest	0	USD
CY2021	us-gaap	Impaired Financing Receivable Interest Income Accrual Method ImpairedFinancingReceivableInterestIncomeAccrualMethod	0	USD
CY2021	us-gaap	Financing Receivable Accrued Interest Writeoff FinancingReceivableAccruedInterestWriteoff	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
CY2021Q4	us-gaap	Long Term Accounts Notes And Loans Receivable Net Noncurrent LongTermAccountsNotesAndLoansReceivableNetNoncurrent	100000	USD
CY2021Q4	us-gaap	Loans And Leases Receivable Impaired Commitment To Lend LoansAndLeasesReceivableImpairedCommitmentToLend	0	USD
CY2020Q4	us-gaap	Financing Receivable Modifications Recorded Investment FinancingReceivableModificationsRecordedInvestment	14500000	USD
CY2020Q4	fibk	Financing Receivable Modifications Recorded Investment Non Accrual FinancingReceivableModificationsRecordedInvestmentNonAccrual	11300000	USD
CY2020Q4	fibk	Financing Receivable Modifications Recorded Investment Accrual Method FinancingReceivableModificationsRecordedInvestmentAccrualMethod	3200000	USD
CY2021Q4	us-gaap	Allowance For Notes And Loans Receivable Noncurrent AllowanceForNotesAndLoansReceivableNoncurrent	2900000	USD
CY2021Q4	us-gaap	Loans And Leases Receivable Impaired Commitment To Lend LoansAndLeasesReceivableImpairedCommitmentToLend	0	USD
CY2021	us-gaap	Financing Receivable Modifications Post Modification Recorded Investment2 FinancingReceivableModificationsPostModificationRecordedInvestment2	0	USD
CY2019	us-gaap	Financing Receivable Modifications Number Of Contracts2 FinancingReceivableModificationsNumberOfContracts2	11	note
CY2019	us-gaap	Financing Receivable Modifications Post Modification Recorded Investment2 FinancingReceivableModificationsPostModificationRecordedInvestment2	10400000	USD
CY2019	us-gaap	Financing Receivables Impaired Troubled Debt Restructuring Write Down FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown	0	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
CY2021	us-gaap	Financing Receivable Modifications Subsequent Default Recorded Investment1 FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1	0	USD
CY2021Q4	us-gaap	Impaired Financing Receivable Recorded Investment ImpairedFinancingReceivableRecordedInvestment	43500000	USD
CY2021	us-gaap	Loans And Leases Receivable Commitments To Purchase Or Sell LoansAndLeasesReceivableCommitmentsToPurchaseOrSell	0	USD
CY2021	us-gaap	Proceeds From Sale Of Loans And Leases Held For Investment ProceedsFromSaleOfLoansAndLeasesHeldForInvestment	0	USD
CY2021Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	495800000	USD
CY2020Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	491300000	USD
CY2021Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	196200000	USD
CY2020Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	179000000.0	USD
CY2021Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	299600000	USD
CY2020Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	312300000	USD
CY2021	us-gaap	Depreciation Depreciation	28000000	USD
CY2020	us-gaap	Depreciation Depreciation	25800000	USD
CY2019	us-gaap	Depreciation Depreciation	23300000	USD
CY2021Q4	fibk	Foreclosed Residential Real Estateincludedin Other Real Estate Owned ForeclosedResidentialRealEstateincludedinOtherRealEstateOwned	200000	USD
CY2021Q4	us-gaap	Life Insurance Corporate Or Bank Owned Amount LifeInsuranceCorporateOrBankOwnedAmount	301500000	USD
CY2020Q4	us-gaap	Life Insurance Corporate Or Bank Owned Amount LifeInsuranceCorporateOrBankOwnedAmount	296400000	USD
CY2020Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	2500000	USD
CY2019Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	8500000	USD
CY2018Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	14400000	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
CY2019	fibk	Real Estate Acquired Through Foreclosure Acquired Through Acquisition RealEstateAcquiredThroughForeclosureAcquiredThroughAcquisition	2400000	USD
CY2021	fibk	Real Estate Acquired Through Foreclosure Additions RealEstateAcquiredThroughForeclosureAdditions	900000	USD
CY2020	fibk	Real Estate Acquired Through Foreclosure Additions RealEstateAcquiredThroughForeclosureAdditions	3300000	USD
CY2019	fibk	Real Estate Acquired Through Foreclosure Additions RealEstateAcquiredThroughForeclosureAdditions	14100000	USD
CY2021	fibk	Real Estate Acquired Through Foreclosure Improvements RealEstateAcquiredThroughForeclosureImprovements	0	USD
CY2020	fibk	Real Estate Acquired Through Foreclosure Improvements RealEstateAcquiredThroughForeclosureImprovements	0	USD
CY2019	fibk	Real Estate Acquired Through Foreclosure Improvements RealEstateAcquiredThroughForeclosureImprovements	300000	USD
CY2021	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	0	USD
CY2020	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	100000	USD
CY2019	us-gaap	Impairment Of Real Estate ImpairmentOfRealEstate	900000	USD
CY2021	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	1400000	USD
CY2020	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	9200000	USD
CY2019	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	21800000	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
CY2021	us-gaap	Impairment Of Real Estate ImpairmentOfRealEstate	0	USD
CY2020	us-gaap	Impairment Of Real Estate ImpairmentOfRealEstate	100000	USD
CY2019	us-gaap	Impairment Of Real Estate ImpairmentOfRealEstate	900000	USD
CY2020Q4	fibk	Foreclosed Residential Real Estateincludedin Other Real Estate Owned ForeclosedResidentialRealEstateincludedinOtherRealEstateOwned	0	USD
CY2021Q4	us-gaap	Mortgage Loans In Process Of Foreclosure Amount MortgageLoansInProcessOfForeclosureAmount	200000	USD
CY2020Q4	us-gaap	Mortgage Loans In Process Of Foreclosure Amount MortgageLoansInProcessOfForeclosureAmount	200000	USD
CY2021Q4	us-gaap	Interest Rate Cash Flow Hedge Derivative At Fair Value Net InterestRateCashFlowHedgeDerivativeAtFairValueNet	0	USD
CY2021Q4	us-gaap	Interest Rate Cash Flow Hedge Gain Loss To Be Reclassified During Next12 Months Net InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet	0	USD
CY2021Q2	us-gaap	Payments For Proceeds From Derivative Instrument Investing Activities PaymentsForProceedsFromDerivativeInstrumentInvestingActivities	500000000	USD
CY2021Q2	us-gaap	Derivatives Basis And Use Of Derivatives DerivativesBasisAndUseOfDerivatives	87
CY2021Q3	us-gaap	Payments For Proceeds From Derivative Instrument Investing Activities PaymentsForProceedsFromDerivativeInstrumentInvestingActivities	200000000	USD
CY2021Q3	us-gaap	Derivatives Basis And Use Of Derivatives DerivativesBasisAndUseOfDerivatives	99
CY2021Q4	us-gaap	Derivative Instruments And Hedges DerivativeInstrumentsAndHedges	695600000	USD
CY2021Q4	us-gaap	Hedged Asset Fair Value Hedge Cumulative Increase Decrease HedgedAssetFairValueHedgeCumulativeIncreaseDecrease	-4400000	USD
CY2020Q4	us-gaap	Derivative Instruments And Hedges DerivativeInstrumentsAndHedges	0	USD
CY2020Q4	us-gaap	Hedged Asset Fair Value Hedge Cumulative Increase Decrease HedgedAssetFairValueHedgeCumulativeIncreaseDecrease	0	USD
CY2021Q4	us-gaap	Derivative Asset Notional Amount DerivativeAssetNotionalAmount	1691200000	USD
CY2021Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	28100000	USD
CY2020Q4	us-gaap	Derivative Asset Notional Amount DerivativeAssetNotionalAmount	901600000	USD
CY2020Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	55300000	USD
CY2021Q4	us-gaap	Derivative Liability Notional Amount DerivativeLiabilityNotionalAmount	1103900000	USD
CY2021Q4	us-gaap	Interest Rate Derivative Liabilities At Fair Value InterestRateDerivativeLiabilitiesAtFairValue	18200000	USD
CY2020Q4	us-gaap	Derivative Liability Notional Amount DerivativeLiabilityNotionalAmount	1014100000	USD
CY2020Q4	us-gaap	Interest Rate Derivative Liabilities At Fair Value InterestRateDerivativeLiabilitiesAtFairValue	17900000	USD
CY2021	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationTax	0	USD
CY2021	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationTax	0	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 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 Asset DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset	0	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
CY2020Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	55300000	USD
CY2020Q4	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	USD
CY2020Q4	us-gaap	Derivative Assets DerivativeAssets	55300000	USD
CY2020Q4	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	USD
CY2020Q4	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	17200000	USD
CY2020Q4	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	38100000	USD
CY2020Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	55300000	USD
CY2020Q4	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	USD
CY2020Q4	us-gaap	Derivative Assets DerivativeAssets	55300000	USD
CY2020Q4	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	USD
CY2020Q4	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	17200000	USD
CY2020Q4	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	38100000	USD
CY2020Q4	us-gaap	Derivative Fair Value Of Derivative Liability DerivativeFairValueOfDerivativeLiability	17900000	USD
CY2020Q4	us-gaap	Derivative Liability Fair Value Gross Asset DerivativeLiabilityFairValueGrossAsset	0	USD
CY2020Q4	us-gaap	Derivative Liabilities DerivativeLiabilities	17900000	USD
CY2020Q4	us-gaap	Derivative Collateral Right To Reclaim Securities DerivativeCollateralRightToReclaimSecurities	0	USD
CY2020Q4	us-gaap	Derivative Collateral Right To Reclaim Cash DerivativeCollateralRightToReclaimCash	0	USD
CY2020Q4	us-gaap	Derivative Fair Value Of Derivative Liability Amount Offset Against Collateral DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral	17900000	USD
CY2020Q4	us-gaap	Securities Sold Under Agreements To Repurchase Gross SecuritiesSoldUnderAgreementsToRepurchaseGross	1091400000	USD
CY2020Q4	us-gaap	Securities Sold Under Agreements To Repurchase Asset SecuritiesSoldUnderAgreementsToRepurchaseAsset	0	USD
CY2020Q4	us-gaap	Securities Sold Under Agreements To Repurchase SecuritiesSoldUnderAgreementsToRepurchase	1091400000	USD
CY2020Q4	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Securities SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimSecurities	0	USD
CY2020Q4	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Cash SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimCash	1091400000	USD
CY2020Q4	us-gaap	Securities Sold Under Agreements To Repurchase Amount Offset Against Collateral SecuritiesSoldUnderAgreementsToRepurchaseAmountOffsetAgainstCollateral	0	USD
CY2020Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Gross DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedGross	1109300000	USD
CY2020Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Asset DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset	0	USD
CY2020Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoaned	1109300000	USD
CY2020Q4	us-gaap	Deposits Savings Deposits DepositsSavingsDeposits	4405900000	USD
CY2020Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Securities DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimSecurities	0	USD
CY2020Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Cash DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimCash	1091400000	USD
CY2020Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Amount Offset Against Collateral DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAmountOffsetAgainstCollateral	17900000	USD
CY2021Q4	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	5568300000	USD
CY2020Q4	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	4633500000	USD
CY2021Q4	us-gaap	Demand Deposit Accounts DemandDepositAccounts	4753200000	USD
CY2020Q4	us-gaap	Demand Deposit Accounts DemandDepositAccounts	4118900000	USD
CY2021Q4	us-gaap	Deposits Savings Deposits DepositsSavingsDeposits	4981600000	USD
CY2021Q4	us-gaap	Time Deposits100000 Or More TimeDeposits100000OrMore	186700000	USD
CY2020Q4	us-gaap	Time Deposits100000 Or More TimeDeposits100000OrMore	193000000.0	USD
CY2021Q4	us-gaap	Time Deposits Less Than100000 TimeDepositsLessThan100000	779800000	USD
CY2020Q4	us-gaap	Time Deposits Less Than100000 TimeDepositsLessThan100000	865700000	USD
CY2021Q4	us-gaap	Interest Bearing Deposit Liabilities InterestBearingDepositLiabilities	10701300000	USD
CY2020Q4	us-gaap	Interest Bearing Deposit Liabilities InterestBearingDepositLiabilities	9583500000	USD
CY2021Q4	us-gaap	Deposits Deposits	16269600000	USD
CY2020Q4	us-gaap	Deposits Deposits	14217000000	USD
CY2021Q4	fibk	Fdic Deposit Insurance Limit FDICDepositInsuranceLimit	250000000	USD
CY2021Q4	fibk	Time Deposits Obtained Certificate Of Deposit Account Registry Service TimeDepositsObtainedCertificateOfDepositAccountRegistryService	104500000	USD
CY2020Q4	fibk	Time Deposits Obtained Certificate Of Deposit Account Registry Service TimeDepositsObtainedCertificateOfDepositAccountRegistryService	97300000	USD
CY2021Q4	us-gaap	Interest Bearing Domestic Deposit Brokered InterestBearingDomesticDepositBrokered	0	USD
CY2021Q4	fibk	Time Deposits250000or More TimeDeposits250000orMore	186700000	USD
CY2020Q4	fibk	Time Deposits250000or More TimeDeposits250000orMore	193000000	USD
CY2021Q4	fibk	Fdic Deposit Insurance Limit FDICDepositInsuranceLimit	250000000	USD
CY2021Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More Three Months Or Less ContractualMaturitiesTimeDeposits100000OrMoreThreeMonthsOrLess	38800000	USD
CY2021Q4	fibk	Contractual Maturities Time Deposits Three Monthsor Less ContractualMaturitiesTimeDepositsThreeMonthsorLess	272700000	USD
CY2021Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More Three Months Through Six Months ContractualMaturitiesTimeDeposits100000OrMoreThreeMonthsThroughSixMonths	47200000	USD
CY2021Q4	fibk	Contractual Maturities Time Deposits Three Monthsthrough Six Months ContractualMaturitiesTimeDepositsThreeMonthsthroughSixMonths	209700000	USD
CY2021Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More Six Months Through12 Months ContractualMaturitiesTimeDeposits100000OrMoreSixMonthsThrough12Months	70600000	USD
CY2021Q4	fibk	Contractual Maturities Time Deposits Six Monthsthrough Twelve Months ContractualMaturitiesTimeDepositsSixMonthsthroughTwelveMonths	293700000	USD
CY2021Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More After12 Months ContractualMaturitiesTimeDeposits100000OrMoreAfter12Months	18100000	USD
CY2021Q4	us-gaap	Time Deposits Noncurrent TimeDepositsNoncurrent	100600000	USD
CY2021Q4	fibk	Contractual Maturities Time Deposits100000or Moreafter2years ContractualMaturitiesTimeDeposits100000orMoreafter2years	4500000	USD
CY2021Q4	us-gaap	Time Deposit Maturities Year Two TimeDepositMaturitiesYearTwo	43700000	USD
CY2021Q4	fibk	Contractual Maturities Time Deposits100000or Moreafter3years ContractualMaturitiesTimeDeposits100000orMoreafter3years	4400000	USD
CY2021Q4	us-gaap	Time Deposit Maturities Year Three TimeDepositMaturitiesYearThree	28700000	USD
CY2021Q4	fibk	Contractual Maturities Time Deposits100000 Or More After4 Years ContractualMaturitiesTimeDeposits100000OrMoreAfter4Years	3100000	USD
CY2021Q4	us-gaap	Time Deposit Maturities After Year Five TimeDepositMaturitiesAfterYearFive	17400000	USD
CY2021Q4	us-gaap	Time Deposits100000 Or More TimeDeposits100000OrMore	186700000	USD
CY2021Q4	us-gaap	Time Deposits TimeDeposits	966500000	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
CY2019	us-gaap	Interest Expense Time Deposits100000 Or More InterestExpenseTimeDeposits100000OrMore	4600000	USD
CY2020Q2	us-gaap	Debt Instrument Interest Rate Stated Percentage DebtInstrumentInterestRateStatedPercentage	0.0525
CY2021Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	112400000	USD
CY2020Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	112400000	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	5200000	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	100000	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	100000	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Four LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour	100000	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive	100000	USD
CY2021Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal After Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive	106800000	USD
CY2021Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	112400000	USD
CY2021Q4	us-gaap	Advances From Federal Home Loan Banks AdvancesFromFederalHomeLoanBanks	0	USD
CY2020Q2	us-gaap	Debt Instrument Interest Rate Stated Percentage DebtInstrumentInterestRateStatedPercentage	0.0525
CY2021Q4	us-gaap	Debt Instrument Unamortized Discount Premium And Debt Issuance Costs Net DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet	1300000	USD
CY2021Q4	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	0	USD
CY2021	fibk	Company Sponsored Wholly Owned Business Trusts Number CompanySponsoredWhollyOwnedBusinessTrustsNumber	7	trusts
CY2021Q4	us-gaap	Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust	87000000.0	USD
CY2020Q4	us-gaap	Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust	87000000.0	USD
CY2021Q4	us-gaap	Debt Instrument Interest Rate Effective Percentage DebtInstrumentInterestRateEffectivePercentage	0.0288
CY2021Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	200000000	shares
CY2021	us-gaap	Incremental Common Shares Attributable To Accelerated Share Repurchase Agreements IncrementalCommonSharesAttributableToAcceleratedShareRepurchaseAgreements	3000000	shares
CY2020Q3	us-gaap	Stock Repurchase Program Number Of Shares Authorized To Be Repurchased StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased	5500000	shares
CY2021Q4	fibk	Sale Of Stock Shares Authorized For Future Sale SaleOfStockSharesAuthorizedForFutureSale	47158390	shares
CY2020	us-gaap	Net Income Loss NetIncomeLoss	161200000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	181000000.0	USD
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
CY2019	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	63645029	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
CY2019	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	239839	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
CY2019	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	63884868	shares
CY2021	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	3.12
CY2020	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.53
CY2019	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.84
CY2021	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	3.11
CY2020	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.53
CY2019	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.83
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
CY2020Q4	us-gaap	Capital Capital	1575700000	USD
CY2020Q4	us-gaap	Capital To Risk Weighted Assets CapitalToRiskWeightedAssets	0.1419
CY2020Q4	us-gaap	Capital Required For Capital Adequacy CapitalRequiredForCapitalAdequacy	888300000	USD
CY2020Q4	us-gaap	Capital Required For Capital Adequacy To Risk Weighted Assets CapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0800
CY2020Q4	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	1165800000	USD
CY2020Q4	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.1050
CY2020Q4	us-gaap	Capital Required To Be Well Capitalized CapitalRequiredToBeWellCapitalized	1110300000	USD
CY2020Q4	us-gaap	Capital Required To Be Well Capitalized To Risk Weighted Assets CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.1000
CY2020Q4	us-gaap	Tier One Risk Based Capital TierOneRiskBasedCapital	1369000000	USD
CY2020Q4	us-gaap	Tier One Risk Based Capital To Risk Weighted Assets TierOneRiskBasedCapitalToRiskWeightedAssets	0.1233
CY2020Q4	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy TierOneRiskBasedCapitalRequiredForCapitalAdequacy	666200000	USD
CY2020Q4	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0600
CY2020Q4	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	943800000	USD
CY2020Q4	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0850
CY2020Q4	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized TierOneRiskBasedCapitalRequiredToBeWellCapitalized	888300000	USD
CY2020Q4	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.0800
CY2020Q4	fibk	Common Equity Tier One Risked Based Capital CommonEquityTierOneRiskedBasedCapital	1284900000	USD
CY2020Q4	fibk	Common Equity Tier One Risk Based Capital To Risk Weighted Assets CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets	0.1157
CY2020Q4	fibk	Common Equity Tier One Capital Required For Capital Adequacy Under Basel Iii Phase In Schedule CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInSchedule	499600000	USD
CY2020Q4	fibk	Common Equity Tier One Capital Required For Capital Adequacy Under Basel Iii Phase In Schedule To Risk Weighted Assets CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInScheduleToRiskWeightedAssets	0.0450
CY2020Q4	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	777200000	USD
CY2020Q4	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0700
CY2020Q4	fibk	Common Equity Tier One Capital To Be Well Capitalized CommonEquityTierOneCapitalToBeWellCapitalized	721700000	USD
CY2020Q4	fibk	Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets CommonEquityTierOneCapitalToBeWellCapitalizedToRiskWeightedAssets	0.0650
CY2020Q4	us-gaap	Tier One Leverage Capital TierOneLeverageCapital	1369000000	USD
CY2020Q4	us-gaap	Tier One Leverage Capital To Average Assets TierOneLeverageCapitalToAverageAssets	0.0816
CY2020Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	671000000.0	USD
CY2020Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2020Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	671000000.0	USD
CY2020Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2020Q4	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized TierOneLeverageCapitalRequiredToBeWellCapitalized	838700000	USD
CY2020Q4	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized To Average Assets TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets	0.0500
CY2021	us-gaap	Operating Lease Expense OperatingLeaseExpense	4300000	USD
CY2020	us-gaap	Operating Lease Expense OperatingLeaseExpense	5100000	USD
CY2019	us-gaap	Lease And Rental Expense LeaseAndRentalExpense	4800000	USD
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CY2021	us-gaap	Employee Service Share Based Compensation Tax Benefit From Exercise Of Stock Options EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions	200000	USD
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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
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CY2021	us-gaap	Other Comprehensive Income Loss Transfers From Held To Maturity To Available For Sale Securities Net Of Tax OtherComprehensiveIncomeLossTransfersFromHeldToMaturityToAvailableForSaleSecuritiesNetOfTax	-15100000	USD
CY2021	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments Before Tax Noncontrolling Interest1 OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsBeforeTaxNoncontrollingInterest1	4200000	USD
CY2021	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease Adjustments Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAdjustmentsTax	1100000	USD
CY2021	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsTax	3100000	USD
CY2021Q1	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-90400000	USD
CY2021Q1	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-22800000	USD
CY2021Q1	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 Derivative Excluded Component Increase Decrease After Adjustments Before Tax Noncontrolling Interest1 OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsBeforeTaxNoncontrollingInterest1	200000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease Adjustments Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAdjustmentsTax	0	USD
CY2020	us-gaap	Other Comprehensive Income Loss Derivative Excluded Component Increase Decrease After Adjustments Tax OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsTax	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
CY2019	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	54900000	USD
CY2019	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	14100000	USD
CY2019	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	40800000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	100000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax	0	USD
CY2019	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Net Of Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	100000	USD
CY2019	fibk	Other Comprehensive Income Loss Transfers From Available For Sale To Held To Maturity Securities Before Tax OtherComprehensiveIncomeLossTransfersFromAvailableForSaleToHeldToMaturitySecuritiesBeforeTax	-6000000.0	USD
CY2019	fibk	Other Comprehensive Income Loss Transfers From Available For Sale To Held To Maturity Securities Tax OtherComprehensiveIncomeLossTransfersFromAvailableForSaleToHeldToMaturitySecuritiesTax	-1600000	USD
CY2019	fibk	Other Comprehensive Income Loss Transfers From Available For Sale To Held To Maturity Securities Net Of Tax OtherComprehensiveIncomeLossTransfersFromAvailableForSaleToHeldToMaturitySecuritiesNetOfTax	-4400000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Net Unamortized Gain Loss Arising During Period Before Tax OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax	-800000	USD
CY2019	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodTax	-100000	USD
CY2019	us-gaap	Other Comprehensive Income Defined Benefit Plans Net Unamortized Gain Loss Arising During Period Net Of Tax OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax	-700000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Before Tax Portion Attributable To Parent OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParent	48000000.0	USD
CY2019	us-gaap	Other Comprehensive Income Loss Tax Portion Attributable To Parent1 OtherComprehensiveIncomeLossTaxPortionAttributableToParent1	12400000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	35600000	USD
CY2021	us-gaap	Held To Maturity Securities Transferred Security Unrealized Gain Loss HeldToMaturitySecuritiesTransferredSecurityUnrealizedGainLoss	15000000.0	USD
CY2020	us-gaap	Held To Maturity Securities Transferred Security Unrealized Gain Loss HeldToMaturitySecuritiesTransferredSecurityUnrealizedGainLoss	0	USD
CY2021	fibk	Net Unrealized Gain Loss On Derivatives Net Of Tax NetUnrealizedGainLossOnDerivativesNetOfTax	3000000.0	USD
CY2020	fibk	Net Unrealized Gain Loss On Derivatives Net Of Tax NetUnrealizedGainLossOnDerivativesNetOfTax	-200000	USD
CY2021Q4	us-gaap	Long Term Debt LongTermDebt	98700000	USD
CY2020Q4	us-gaap	Long Term Debt LongTermDebt	98600000	USD
CY2021	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
CY2020	us-gaap	Loans And Leases Receivable Related Parties Proceeds LoansAndLeasesReceivableRelatedPartiesProceeds	700000	USD
CY2019	us-gaap	Loans And Leases Receivable Related Parties Proceeds LoansAndLeasesReceivableRelatedPartiesProceeds	500000	USD

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$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

2021 Q4 Form 10-K Financial Statement

#000086041322000065 Filed on February 25, 2022

Income Statement

Balance Sheet

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$FIBK FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

2021 Q4 Form 10-K Financial Statement

#000086041322000065 Filed on February 25, 2022

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk