$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

$fibk Overview Financials Valuation SEC Filings Company Website

2024 Q1 Form 10-K Financial Statement

#000086041324000016 Filed on February 29, 2024

Income Statement

Concept	2024 Q1	2023 Q4	2023
Revenue	$324.7M		$1.280B
YoY Change	2.79%		25.32%
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	$15.90M	$13.30M	$53.80M
YoY Change	16.06%	-5.67%	-3.06%
% of Gross Profit
Operating Expenses
YoY Change
Operating Profit
YoY Change
Interest Expense	$124.6M		$401.3M
YoY Change	61.82%		408.62%
% of Operating Profit
Other Income/Expense, Net
YoY Change
Pretax Income	$76.70M	$80.90M	$336.8M
YoY Change	3.23%	-26.45%	31.0%
Income Tax	$18.30M	$19.40M	$79.30M
% Of Pretax Income	23.86%	23.98%	23.55%
Net Earnings	$58.40M	$61.50M	$257.5M
YoY Change	3.73%	-28.32%	27.35%
Net Earnings / Revenue	17.99%		20.12%
Basic Earnings Per Share	$0.57		$2.48
Diluted Earnings Per Share	$0.57	$0.59	$2.48
COMMON SHARES
Basic Shares Outstanding	103.9M shares	105.0M shares	103.8M shares
Diluted Shares Outstanding	103.0M shares		103.8M shares

Balance Sheet

Concept	2024 Q1	2023 Q4	2023
SHORT-TERM ASSETS
Cash & Short-Term Investments		$579.2M
YoY Change		-33.56%
Cash & Equivalents	$635.0M	$578.0M
Short-Term Investments		$1.200M
Other Short-Term Assets
YoY Change
Inventory
Prepaid Expenses
Receivables
Other Receivables
Total Short-Term Assets
YoY Change
LONG-TERM ASSETS
Property, Plant & Equipment	$439.9M	$444.3M	$444.3M
YoY Change	-0.79%	-0.09%	-0.09%
Goodwill	$1.101B	$1.101B
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	$30.14B	$30.67B	$30.67B
YoY Change	-4.72%	-5.01%	-5.01%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable
YoY Change
Accrued Expenses
YoY Change
Deferred Revenue
YoY Change
Short-Term Debt		$3.386B	$3.386B
YoY Change		0.17%	0.17%
Long-Term Debt Due
YoY Change
Total Short-Term Liabilities		$3.386B	$3.386B
YoY Change		635.38%	635.38%
LONG-TERM LIABILITIES
Long-Term Debt		$283.9M	$283.9M
YoY Change		0.0%	0.0%
Other Long-Term Liabilities		$451.0M	$451.0M
YoY Change		-5.37%	-5.37%
Total Long-Term Liabilities		$734.9M	$734.9M
YoY Change		-3.37%	-3.37%
TOTAL LIABILITIES
Total Short-Term Liabilities		$3.386B	$3.386B
Total Long-Term Liabilities		$734.9M	$734.9M
Total Liabilities	$26.94B	$27.44B	$27.44B
YoY Change	-5.42%	-6.06%	-6.06%
SHAREHOLDERS EQUITY
Retained Earnings	$1.146B	$1.135B
YoY Change	6.03%	5.82%
Common Stock		$0.00
YoY Change
Preferred Stock
YoY Change
Treasury Stock (at cost)
YoY Change
Treasury Stock Shares
Shareholders Equity	$3.210B	$3.228B	$3.228B
YoY Change
Total Liabilities & Shareholders Equity	$30.14B	$30.67B	$30.67B
YoY Change	-4.72%	-5.01%	-5.01%

Cashflow Statement

Concept	2024 Q1	2023 Q4	2023
OPERATING ACTIVITIES
Net Income	$58.40M	$61.50M	$257.5M
YoY Change	3.73%	-28.32%	27.35%
Depreciation, Depletion And Amortization	$15.90M	$13.30M	$53.80M
YoY Change	16.06%	-5.67%	-3.06%
Cash From Operating Activities	$87.60M	$69.30M	$428.0M
YoY Change	51.82%	-27.66%	-19.91%
INVESTING ACTIVITIES
Capital Expenditures		$8.800M	$28.20M
YoY Change		-152.69%	168.57%
Acquisitions
YoY Change
Other Investing Activities		-$67.50M	$1.282B
YoY Change		-89.66%	-236.56%
Cash From Investing Activities	$531.9M	-$76.30M	$1.253B
YoY Change	-38.74%	-88.0%	-232.08%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net
YoY Change
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	-$562.5M	-8.100M	-$1.974B
YoY Change	-21.42%	-100.99%	86.27%
NET CHANGE
Cash From Operating Activities	$87.60M	69.30M	$428.0M
Cash From Investing Activities	$531.9M	-76.30M	$1.253B
Cash From Financing Activities	-$562.5M	-8.100M	-$1.974B
Net Change In Cash	$57.00M	-15.10M	-$292.5M
YoY Change	-72.88%	-105.42%	-80.16%
FREE CASH FLOW
Cash From Operating Activities	$87.60M	$69.30M	$428.0M
Capital Expenditures		$8.800M	$28.20M
Free Cash Flow		$60.50M	$399.8M
YoY Change		-46.22%	-23.69%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2023	fibk	Payment Services Revenues PaymentServicesRevenues	76400000	usd
CY2023	us-gaap	Fees And Commissions Mortgage Banking And Servicing FeesAndCommissionsMortgageBankingAndServicing	8400000	usd
CY2022	us-gaap	Fees And Commissions Mortgage Banking And Servicing FeesAndCommissionsMortgageBankingAndServicing	18700000	usd
CY2023	fibk	Service Charges On Deposit Accounts ServiceChargesOnDepositAccounts	23000000.0	usd
CY2022	fibk	Service Charges On Deposit Accounts ServiceChargesOnDepositAccounts	24600000	usd
CY2022	fibk	Other Service Charges Commissions And Fees OtherServiceChargesCommissionsAndFees	15500000	usd
CY2021	us-gaap	Debt Securities Available For Sale Gain Loss DebtSecuritiesAvailableForSaleGainLoss	1100000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Allowance For Credit Loss Excluding Accrued Interest DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest	0	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Excluding Accrued Interest After Allowance For Credit Loss DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss	6307800000	usd
CY2023Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	3200000	usd
CY2023Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	469500000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	5841500000	usd
CY2021	fibk	Other Service Charges Commissions And Fees OtherServiceChargesCommissionsAndFees	7900000	usd
CY2022	us-gaap	Debt Securities Available For Sale Gain Loss DebtSecuritiesAvailableForSaleGainLoss	-24400000	usd
CY2023	dei	Entity Central Index Key EntityCentralIndexKey	0000860413
CY2023	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2023	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2023
CY2023	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
CY2023	dei	Amendment Flag AmendmentFlag	false
CY2023	dei	Document Annual Report DocumentAnnualReport	true
CY2023	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	Yes
CY2023	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
CY2023	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2023	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
CY2023	dei	Entity Filer Category EntityFilerCategory	Large Accelerated Filer
CY2023	dei	Entity Small Business EntitySmallBusiness	false
CY2023	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
CY2023	dei	Icfr Auditor Attestation Flag IcfrAuditorAttestationFlag	true
CY2023	dei	Document Fin Stmt Error Correction Flag DocumentFinStmtErrorCorrectionFlag	false
CY2023	dei	Entity Shell Company EntityShellCompany	false
CY2023Q2	dei	Entity Public Float EntityPublicFloat	2100000000	usd
CY2024Q1	dei	Entity Common Stock Shares Outstanding EntityCommonStockSharesOutstanding	103935844	shares
CY2023Q4	us-gaap	Goodwill Goodwill	1100900000	usd
CY2023Q3	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	0	usd
CY2022Q3	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	0	usd
CY2023	dei	Auditor Firm AuditorFirmId	49
CY2023Q4	us-gaap	Loans And Leases Receivable Net Of Deferred Income LoansAndLeasesReceivableNetOfDeferredIncome	18279600000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Allowance LoansAndLeasesReceivableAllowance	227700000	usd
CY2023Q4	us-gaap	Goodwill Goodwill	1100900000	usd
CY2023	dei	Auditor Name AuditorName	RSM US LLP
CY2023	dei	Auditor Location AuditorLocation	Des Moines, Iowa
CY2023Q4	us-gaap	Cash And Due From Banks CashAndDueFromBanks	378200000	usd
CY2022Q4	us-gaap	Cash And Due From Banks CashAndDueFromBanks	349200000	usd
CY2023Q4	us-gaap	Interest Bearing Deposits In Banks InterestBearingDepositsInBanks	199700000	usd
CY2022Q4	us-gaap	Interest Bearing Deposits In Banks InterestBearingDepositsInBanks	521200000	usd
CY2023Q4	us-gaap	Federal Funds Sold FederalFundsSold	100000	usd
CY2022Q4	us-gaap	Federal Funds Sold FederalFundsSold	100000	usd
CY2023Q4	us-gaap	Cash Cash Equivalents And Federal Funds Sold CashCashEquivalentsAndFederalFundsSold	578000000.0	usd
CY2022Q4	us-gaap	Cash Cash Equivalents And Federal Funds Sold CashCashEquivalentsAndFederalFundsSold	870500000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Allowance For Credit Loss Excluding Accrued Interest DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest	0	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Excluding Accrued Interest After Allowance For Credit Loss DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss	6307800000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Excluding Accrued Interest Before Allowance For Credit Loss DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss	7573500000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	5841500000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	6946100000	usd
CY2023Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	800000	usd
CY2022Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	1900000	usd
CY2023Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	2874000000	usd
CY2022Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	3052200000	usd
CY2023Q4	us-gaap	Debt Securities Held To Maturity Amortized Cost After Allowance For Credit Loss DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss	3207900000	usd
CY2022Q4	us-gaap	Debt Securities Held To Maturity Amortized Cost After Allowance For Credit Loss DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss	3451800000	usd
CY2023Q4	us-gaap	Investments Investments	9049400000	usd
CY2022Q4	us-gaap	Investments Investments	10397900000	usd
CY2023Q4	us-gaap	Federal Home Loan Bank Stock And Federal Reserve Bank Stock FederalHomeLoanBankStockAndFederalReserveBankStock	223200000	usd
CY2022Q4	us-gaap	Federal Home Loan Bank Stock And Federal Reserve Bank Stock FederalHomeLoanBankStockAndFederalReserveBankStock	198600000	usd
CY2023Q4	us-gaap	Mortgages Held For Sale Fair Value Disclosure MortgagesHeldForSaleFairValueDisclosure	47400000	usd
CY2022Q4	us-gaap	Mortgages Held For Sale Fair Value Disclosure MortgagesHeldForSaleFairValueDisclosure	79900000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Net Of Deferred Income LoansAndLeasesReceivableNetOfDeferredIncome	18279600000	usd
CY2022Q4	us-gaap	Loans And Leases Receivable Net Of Deferred Income LoansAndLeasesReceivableNetOfDeferredIncome	18099200000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Allowance LoansAndLeasesReceivableAllowance	227700000	usd
CY2022Q4	us-gaap	Loans And Leases Receivable Allowance LoansAndLeasesReceivableAllowance	220100000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Net Reported Amount LoansAndLeasesReceivableNetReportedAmount	18051900000	usd
CY2022Q4	us-gaap	Loans And Leases Receivable Net Reported Amount LoansAndLeasesReceivableNetReportedAmount	17879100000	usd
CY2023Q4	us-gaap	Goodwill Goodwill	1100900000	usd
CY2022Q4	us-gaap	Goodwill Goodwill	1100900000	usd
CY2023Q4	us-gaap	Bank Owned Life Insurance BankOwnedLifeInsurance	502400000	usd
CY2022Q4	us-gaap	Bank Owned Life Insurance BankOwnedLifeInsurance	497900000	usd
CY2023Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	444300000	usd
CY2022Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	444700000	usd
CY2023Q4	us-gaap	Indefinite Lived Intangible Assets Excluding Goodwill IndefiniteLivedIntangibleAssetsExcludingGoodwill	81400000	usd
CY2022Q4	us-gaap	Indefinite Lived Intangible Assets Excluding Goodwill IndefiniteLivedIntangibleAssetsExcludingGoodwill	97000000.0	usd
CY2023Q4	us-gaap	Interest Receivable InterestReceivable	129100000	usd
CY2022Q4	us-gaap	Interest Receivable InterestReceivable	118300000	usd
CY2023Q4	us-gaap	Servicing Asset At Amortized Value ServicingAssetAtAmortizedValue	28300000	usd
CY2022Q4	us-gaap	Servicing Asset At Amortized Value ServicingAssetAtAmortizedValue	31100000	usd
CY2023Q4	us-gaap	Other Real Estate OtherRealEstate	16500000	usd
CY2022Q4	us-gaap	Other Real Estate OtherRealEstate	12700000	usd
CY2023Q4	us-gaap	Deferred Income Tax Assets Net DeferredIncomeTaxAssetsNet	150000000.0	usd
CY2022Q4	us-gaap	Deferred Income Tax Assets Net DeferredIncomeTaxAssetsNet	210500000	usd
CY2023Q4	us-gaap	Other Assets OtherAssets	268400000	usd
CY2022Q4	us-gaap	Other Assets OtherAssets	348700000	usd
CY2023Q4	us-gaap	Assets Assets	30671200000	usd
CY2022Q4	us-gaap	Assets Assets	32287800000	usd
CY2023Q4	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	6029600000	usd
CY2022Q4	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	7560000000	usd
CY2023Q4	us-gaap	Interest Bearing Deposit Liabilities InterestBearingDepositLiabilities	17293500000	usd
CY2022Q4	us-gaap	Interest Bearing Deposit Liabilities InterestBearingDepositLiabilities	17513600000	usd
CY2023Q4	us-gaap	Deposits Deposits	23323100000	usd
CY2022Q4	us-gaap	Deposits Deposits	25073600000	usd
CY2023Q4	us-gaap	Securities Sold Under Agreements To Repurchase SecuritiesSoldUnderAgreementsToRepurchase	782700000	usd
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CY2022Q4	us-gaap	Accounts Payable And Accrued Liabilities Current And Noncurrent AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent	445900000	usd
CY2023Q4	us-gaap	Interest Payable Current And Noncurrent InterestPayableCurrentAndNoncurrent	52200000	usd
CY2022Q4	us-gaap	Interest Payable Current And Noncurrent InterestPayableCurrentAndNoncurrent	14500000	usd
CY2023Q4	us-gaap	Other Long Term Debt OtherLongTermDebt	120800000	usd
CY2022Q4	us-gaap	Other Long Term Debt OtherLongTermDebt	120800000	usd
CY2023Q4	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2603000000	usd
CY2022Q4	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2327000000	usd
CY2023Q4	us-gaap	Allowance For Doubtful Accounts Receivable Current AllowanceForDoubtfulAccountsReceivableCurrent	18400000	usd
CY2022Q4	us-gaap	Allowance For Doubtful Accounts Receivable Current AllowanceForDoubtfulAccountsReceivableCurrent	16200000	usd
CY2023Q4	us-gaap	Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust	163100000	usd
CY2022Q4	us-gaap	Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust	163100000	usd
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CY2022Q4	us-gaap	Liabilities Liabilities	29214000000	usd
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CY2023Q4	us-gaap	Preferred Stock No Par Value PreferredStockNoParValue	0
CY2023Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	100000	shares
CY2022Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	100000	shares
CY2023Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2022Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
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CY2022Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
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CY2023Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	1135100000	usd
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CY2022	us-gaap	Interest Income Securities Tax Exempt InterestIncomeSecuritiesTaxExempt	3900000	usd
CY2021	us-gaap	Interest Income Securities Tax Exempt InterestIncomeSecuritiesTaxExempt	4100000	usd
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CY2022	us-gaap	Interest Income Domestic Deposits InterestIncomeDomesticDeposits	8700000	usd
CY2021	us-gaap	Interest Income Domestic Deposits InterestIncomeDomesticDeposits	2600000	usd
CY2023	us-gaap	Interest And Dividend Income Operating InterestAndDividendIncomeOperating	1280100000	usd
CY2022	us-gaap	Interest And Dividend Income Operating InterestAndDividendIncomeOperating	1021500000	usd
CY2021	us-gaap	Interest And Dividend Income Operating InterestAndDividendIncomeOperating	506500000	usd
CY2023	us-gaap	Interest Expense Deposits InterestExpenseDeposits	242600000	usd
CY2022	us-gaap	Interest Expense Deposits InterestExpenseDeposits	48300000	usd
CY2021	us-gaap	Interest Expense Deposits InterestExpenseDeposits	8100000	usd
CY2023	us-gaap	Interest Expense Securities Sold Under Agreements To Repurchase InterestExpenseSecuritiesSoldUnderAgreementsToRepurchase	6400000	usd
CY2022	us-gaap	Interest Expense Securities Sold Under Agreements To Repurchase InterestExpenseSecuritiesSoldUnderAgreementsToRepurchase	2500000	usd
CY2021	us-gaap	Interest Expense Securities Sold Under Agreements To Repurchase InterestExpenseSecuritiesSoldUnderAgreementsToRepurchase	400000	usd
CY2023	us-gaap	Interest Expense Other Short Term Borrowings InterestExpenseOtherShortTermBorrowings	133800000	usd
CY2022	us-gaap	Interest Expense Other Short Term Borrowings InterestExpenseOtherShortTermBorrowings	15300000	usd
CY2021	us-gaap	Interest Expense Other Short Term Borrowings InterestExpenseOtherShortTermBorrowings	0	usd
CY2023	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	5800000	usd
CY2022	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	6000000.0	usd
CY2021	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	6000000.0	usd
CY2023	us-gaap	Interest Expense Junior Subordinated Debentures InterestExpenseJuniorSubordinatedDebentures	12700000	usd
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CY2021	us-gaap	Interest Expense Junior Subordinated Debentures InterestExpenseJuniorSubordinatedDebentures	2800000	usd
CY2023	us-gaap	Interest Expense InterestExpense	401300000	usd
CY2022	us-gaap	Interest Expense InterestExpense	78900000	usd
CY2021	us-gaap	Interest Expense InterestExpense	17300000	usd
CY2023	us-gaap	Interest Income Expense Net InterestIncomeExpenseNet	878800000	usd
CY2022	us-gaap	Interest Income Expense Net InterestIncomeExpenseNet	942600000	usd
CY2021	us-gaap	Interest Income Expense Net InterestIncomeExpenseNet	489200000	usd
CY2023	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	32200000	usd
CY2022	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	82700000	usd
CY2021	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	-14600000	usd
CY2023	us-gaap	Interest Income Expense After Provision For Loan Loss InterestIncomeExpenseAfterProvisionForLoanLoss	846600000	usd
CY2022	us-gaap	Interest Income Expense After Provision For Loan Loss InterestIncomeExpenseAfterProvisionForLoanLoss	859900000	usd
CY2021	us-gaap	Interest Income Expense After Provision For Loan Loss InterestIncomeExpenseAfterProvisionForLoanLoss	503800000	usd
CY2022	fibk	Payment Services Revenues PaymentServicesRevenues	74100000	usd
CY2021	fibk	Payment Services Revenues PaymentServicesRevenues	45100000	usd
CY2021	us-gaap	Fees And Commissions Mortgage Banking And Servicing FeesAndCommissionsMortgageBankingAndServicing	40800000	usd
CY2023	fibk	Wealth Mangement Revenues WealthMangementRevenues	35300000	usd
CY2022	fibk	Wealth Mangement Revenues WealthMangementRevenues	34300000	usd
CY2021	fibk	Wealth Mangement Revenues WealthMangementRevenues	26300000	usd
CY2021	fibk	Service Charges On Deposit Accounts ServiceChargesOnDepositAccounts	16500000	usd
CY2023	fibk	Other Service Charges Commissions And Fees OtherServiceChargesCommissionsAndFees	9500000	usd
CY2021	us-gaap	Noninterest Income Other Operating Income NoninterestIncomeOtherOperatingIncome	11800000	usd
CY2023	us-gaap	Information Technology And Data Processing InformationTechnologyAndDataProcessing	59000000.0	usd
CY2023	us-gaap	Occupancy Net OccupancyNet	48000000.0	usd
CY2022	us-gaap	Occupancy Net OccupancyNet	44000000.0	usd
CY2022	us-gaap	Unrealized Gain Loss On Derivatives UnrealizedGainLossOnDerivatives	-6700000	usd
CY2021	us-gaap	Unrealized Gain Loss On Derivatives UnrealizedGainLossOnDerivatives	4200000	usd
CY2023	us-gaap	Equipment Expense EquipmentExpense	22100000	usd
CY2022	us-gaap	Equipment Expense EquipmentExpense	23400000	usd
CY2021	us-gaap	Equipment Expense EquipmentExpense	17600000	usd
CY2023	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	1500000	usd
CY2022	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	2300000	usd
CY2022	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	2300000	usd
CY2021	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	-200000	usd
CY2023	us-gaap	Professional Fees ProfessionalFees	19100000	usd
CY2022	us-gaap	Professional Fees ProfessionalFees	19100000	usd
CY2021	us-gaap	Professional Fees ProfessionalFees	12100000	usd
CY2023	us-gaap	Federal Deposit Insurance Corporation Premium Expense FederalDepositInsuranceCorporationPremiumExpense	31500000	usd
CY2022	us-gaap	Federal Deposit Insurance Corporation Premium Expense FederalDepositInsuranceCorporationPremiumExpense	14000000.0	usd
CY2021	us-gaap	Federal Deposit Insurance Corporation Premium Expense FederalDepositInsuranceCorporationPremiumExpense	6600000	usd
CY2023	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	15700000	usd
CY2022	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	15900000	usd
CY2021	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	9900000	usd
CY2023	us-gaap	Other Noninterest Expense OtherNoninterestExpense	121500000	usd
CY2022	us-gaap	Other Noninterest Expense OtherNoninterestExpense	114500000	usd
CY2021	us-gaap	Other Noninterest Expense OtherNoninterestExpense	65700000	usd
CY2023	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	0	usd
CY2022	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	118900000	usd
CY2021	us-gaap	Business Combination Acquisition Related Costs BusinessCombinationAcquisitionRelatedCosts	11600000	usd
CY2023	us-gaap	Noninterest Expense NoninterestExpense	656800000	usd
CY2022	us-gaap	Noninterest Expense NoninterestExpense	766000000.0	usd
CY2021	us-gaap	Noninterest Expense NoninterestExpense	405500000	usd
CY2023	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	336800000	usd
CY2022	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	257100000	usd
CY2021	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	247800000	usd
CY2023	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	79300000	usd
CY2022	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	54900000	usd
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	55700000	usd
CY2023	us-gaap	Net Income Loss NetIncomeLoss	257500000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	202200000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	usd
CY2023	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.48
CY2022	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.96
CY2021	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	3.12
CY2023	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.48
CY2022	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.96
CY2021	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	3.11
CY2023	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	144800000	usd
CY2022	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	-613100000	usd
CY2021	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	-113700000	usd
CY2023	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	-23500000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	-24400000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	1100000	usd
CY2023	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	-7200000	usd
CY2022	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	200000	usd
CY2021	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	0	usd
CY2023	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	160700000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-621300000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-90400000	usd
CY2023	fibk	Oci Debt Securities Available For Sale Transfer To Held To Maturity Gain Loss Before Adjustment And Tax OCIDebtSecuritiesAvailableForSaleTransferToHeldToMaturityGainLossBeforeAdjustmentAndTax	-1300000	usd
CY2022	fibk	Oci Debt Securities Available For Sale Transfer To Held To Maturity Gain Loss Before Adjustment And Tax OCIDebtSecuritiesAvailableForSaleTransferToHeldToMaturityGainLossBeforeAdjustmentAndTax	-26100000	usd
CY2021	fibk	Oci Debt Securities Available For Sale Transfer To Held To Maturity Gain Loss Before Adjustment And Tax OCIDebtSecuritiesAvailableForSaleTransferToHeldToMaturityGainLossBeforeAdjustmentAndTax	20200000	usd
CY2023	us-gaap	Unrealized Gain Loss On Derivatives UnrealizedGainLossOnDerivatives	900000	usd
CY2023	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	40100000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-155200000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-22800000	usd
CY2023	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	120600000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-466100000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-67600000	usd
CY2023	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	378100000	usd
CY2022	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-263900000	usd
CY2021	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	124500000	usd
CY2020Q4	us-gaap	Stockholders Equity StockholdersEquity	1959800000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	usd
CY2021	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-67600000	usd
CY2021	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	128171	shares
CY2021	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	5400000	usd
CY2021	us-gaap	Stock Issued During Period Shares New Issues StockIssuedDuringPeriodSharesNewIssues	19081	shares
CY2021	us-gaap	Stock Issued During Period Value New Issues StockIssuedDuringPeriodValueNewIssues	0	usd
CY2021	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	241307	shares
CY2021Q4	us-gaap	Common Stock Value CommonStockValue	0	usd
CY2021	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Nonvested Options Forfeited Number Of Shares SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedNumberOfShares	73044	shares
CY2021	us-gaap	Stock Issued During Period Value Restricted Stock Award Forfeitures StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0	usd
CY2021	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	45484	shares
CY2021	fibk	Shares Tendered In Payment Of Option Price And Income Tax Withholding Amounts Shares SharesTenderedInPaymentOfOptionPriceAndIncomeTaxWithholdingAmountsShares	6982	shares
CY2021	us-gaap	Stock Issued During Period Value Stock Options Exercised Net Of Tax Benefit Expense StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	400000	usd
CY2021	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	8900000	usd
CY2021	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.64
CY2021	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Stock Options Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	-101600000	usd
CY2021Q4	us-gaap	Stockholders Equity StockholdersEquity	1986600000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	202200000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-466100000	usd
CY2022	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	5040896	shares
CY2022	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	199000000.0	usd
CY2022	us-gaap	Stock Issued During Period Shares New Issues StockIssuedDuringPeriodSharesNewIssues	46913370	shares
CY2022	us-gaap	Stock Issued During Period Value New Issues StockIssuedDuringPeriodValueNewIssues	1722500000	usd
CY2022	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	458177	shares
CY2022Q4	us-gaap	Common Stock Value CommonStockValue	0	usd
CY2022	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Nonvested Options Forfeited Number Of Shares SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedNumberOfShares	106891	shares
CY2022	us-gaap	Stock Issued During Period Value Restricted Stock Award Forfeitures StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0	usd
CY2022	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	17807	shares
CY2022	fibk	Shares Tendered In Payment Of Option Price And Income Tax Withholding Amounts Shares SharesTenderedInPaymentOfOptionPriceAndIncomeTaxWithholdingAmountsShares	4877	shares
CY2022	us-gaap	Stock Issued During Period Value Stock Options Exercised Net Of Tax Benefit Expense StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	100000	usd
CY2022	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	9600000	usd
CY2022	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.70
CY2022	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Stock Options Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	-182100000	usd
CY2022Q4	us-gaap	Stockholders Equity StockholdersEquity	3073800000	usd
CY2023	us-gaap	Net Income Loss NetIncomeLoss	257500000	usd
CY2023	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	120600000	usd
CY2023	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	1056191	shares
CY2023	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	34000000.0	usd
CY2023	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	683574	shares
CY2023Q4	us-gaap	Common Stock Value CommonStockValue	0	usd
CY2023	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Nonvested Options Forfeited Number Of Shares SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedNumberOfShares	127780	shares
CY2023	us-gaap	Stock Issued During Period Value Restricted Stock Award Forfeitures StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0	usd
CY2023	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	4700000	usd
CY2023	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.88
CY2023	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Stock Options Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	-195100000	usd
CY2023Q4	us-gaap	Stockholders Equity StockholdersEquity	3227500000	usd
CY2023	us-gaap	Net Income Loss NetIncomeLoss	257500000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	202200000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	usd
CY2023	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	32200000	usd
CY2022	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	82700000	usd
CY2021	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	-14600000	usd
CY2023	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	-700000	usd
CY2022	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	2000000.0	usd
CY2021	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	1800000	usd
CY2023	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	53800000	usd
CY2022	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	55500000	usd
CY2021	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	44400000	usd
CY2023	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	-2400000	usd
CY2022	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	-17000000.0	usd
CY2021	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	-38800000	usd
CY2023	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	-23500000	usd
CY2022	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	-24400000	usd
CY2021	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	1100000	usd
CY2023	us-gaap	Gain Loss On Sale Of Loans And Leases GainLossOnSaleOfLoansAndLeases	2800000	usd
CY2022	us-gaap	Gain Loss On Sale Of Loans And Leases GainLossOnSaleOfLoansAndLeases	9400000	usd
CY2021	us-gaap	Gain Loss On Sale Of Loans And Leases GainLossOnSaleOfLoansAndLeases	26200000	usd
CY2023	us-gaap	Gains Losses On Sales Of Other Real Estate GainsLossesOnSalesOfOtherRealEstate	100000	usd
CY2022	us-gaap	Gains Losses On Sales Of Other Real Estate GainsLossesOnSalesOfOtherRealEstate	700000	usd
CY2021	us-gaap	Gains Losses On Sales Of Other Real Estate GainsLossesOnSalesOfOtherRealEstate	300000	usd
CY2023	fibk	Impairmentof Real Estateand Other Assets Pending Disposal ImpairmentofRealEstateandOtherAssetsPendingDisposal	1100000	usd
CY2022	fibk	Impairmentof Real Estateand Other Assets Pending Disposal ImpairmentofRealEstateandOtherAssetsPendingDisposal	2800000	usd
CY2021	fibk	Impairmentof Real Estateand Other Assets Pending Disposal ImpairmentofRealEstateandOtherAssetsPendingDisposal	0	usd
CY2023	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	0	usd
CY2022	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	1400000	usd
CY2021	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	0	usd
CY2023	us-gaap	Mortgage Servicing Rights Msr Impairment Recovery MortgageServicingRightsMSRImpairmentRecovery	0	usd
CY2022	us-gaap	Mortgage Servicing Rights Msr Impairment Recovery MortgageServicingRightsMSRImpairmentRecovery	-3400000	usd
CY2021	us-gaap	Mortgage Servicing Rights Msr Impairment Recovery MortgageServicingRightsMSRImpairmentRecovery	-6900000	usd
CY2023	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	20400000	usd
CY2022	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-4700000	usd
CY2021	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	5000000.0	usd
CY2023	us-gaap	Life Insurance Corporate Or Bank Owned Change In Value LifeInsuranceCorporateOrBankOwnedChangeInValue	9700000	usd
CY2022	us-gaap	Life Insurance Corporate Or Bank Owned Change In Value LifeInsuranceCorporateOrBankOwnedChangeInValue	11100000	usd
CY2021	us-gaap	Life Insurance Corporate Or Bank Owned Change In Value LifeInsuranceCorporateOrBankOwnedChangeInValue	6100000	usd
CY2023	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	351500000	usd
CY2022	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	429500000	usd
CY2021	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	817400000	usd
CY2023	us-gaap	Proceeds From Sale Of Mortgage Loans Held For Sale ProceedsFromSaleOfMortgageLoansHeldForSale	359500000	usd
CY2022	us-gaap	Proceeds From Sale Of Mortgage Loans Held For Sale ProceedsFromSaleOfMortgageLoansHeldForSale	461600000	usd
CY2021	us-gaap	Proceeds From Sale Of Mortgage Loans Held For Sale ProceedsFromSaleOfMortgageLoansHeldForSale	883900000	usd
CY2023	us-gaap	Increase Decrease In Accrued Interest Receivable Net IncreaseDecreaseInAccruedInterestReceivableNet	10800000	usd
CY2022	us-gaap	Increase Decrease In Accrued Interest Receivable Net IncreaseDecreaseInAccruedInterestReceivableNet	37800000	usd
CY2021	us-gaap	Increase Decrease In Accrued Interest Receivable Net IncreaseDecreaseInAccruedInterestReceivableNet	-3700000	usd
CY2023	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	-80500000	usd
CY2022	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	12900000	usd
CY2021	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	16400000	usd
CY2023	us-gaap	Increase Decrease In Interest Payable Net IncreaseDecreaseInInterestPayableNet	37700000	usd
CY2022	us-gaap	Increase Decrease In Interest Payable Net IncreaseDecreaseInInterestPayableNet	8400000	usd
CY2021	us-gaap	Increase Decrease In Interest Payable Net IncreaseDecreaseInInterestPayableNet	-2000000.0	usd
CY2023	us-gaap	Increase Decrease In Accounts Payable And Accrued Liabilities IncreaseDecreaseInAccountsPayableAndAccruedLiabilities	-71100000	usd
CY2022	us-gaap	Increase Decrease In Accounts Payable And Accrued Liabilities IncreaseDecreaseInAccountsPayableAndAccruedLiabilities	183100000	usd
CY2021	us-gaap	Increase Decrease In Accounts Payable And Accrued Liabilities IncreaseDecreaseInAccountsPayableAndAccruedLiabilities	-1700000	usd
CY2023	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	428000000.0	usd
CY2022	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	534400000	usd
CY2021	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	282300000	usd
CY2023	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	500000	usd
CY2022	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	858500000	usd
CY2021	us-gaap	Payments To Acquire Held To Maturity Securities PaymentsToAcquireHeldToMaturitySecurities	1238000000	usd
CY2023	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	134200000	usd
CY2022	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	3309400000	usd
CY2021	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	2717800000	usd
CY2023	us-gaap	Proceeds From Sale And Maturity Of Held To Maturity Securities ProceedsFromSaleAndMaturityOfHeldToMaturitySecurities	227300000	usd
CY2022	us-gaap	Proceeds From Sale And Maturity Of Held To Maturity Securities ProceedsFromSaleAndMaturityOfHeldToMaturitySecurities	370500000	usd
CY2021	us-gaap	Proceeds From Sale And Maturity Of Held To Maturity Securities ProceedsFromSaleAndMaturityOfHeldToMaturitySecurities	257600000	usd
CY2023	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	1390700000	usd
CY2022	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	1926500000	usd
CY2021	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	1118000000	usd
CY2023	fibk	Purchases Of Federal Home Loan Bank And Federal Reserve Bank Stock PurchasesOfFederalHomeLoanBankAndFederalReserveBankStock	290800000	usd
CY2022	fibk	Purchases Of Federal Home Loan Bank And Federal Reserve Bank Stock PurchasesOfFederalHomeLoanBankAndFederalReserveBankStock	241400000	usd
CY2021	fibk	Purchases Of Federal Home Loan Bank And Federal Reserve Bank Stock PurchasesOfFederalHomeLoanBankAndFederalReserveBankStock	0	usd
CY2023	fibk	Proceeds From Federal Home Loan Bank And Federal Reserve Bank Stock ProceedsFromFederalHomeLoanBankAndFederalReserveBankStock	266200000	usd
CY2022	fibk	Proceeds From Federal Home Loan Bank And Federal Reserve Bank Stock ProceedsFromFederalHomeLoanBankAndFederalReserveBankStock	112200000	usd
CY2021	fibk	Proceeds From Federal Home Loan Bank And Federal Reserve Bank Stock ProceedsFromFederalHomeLoanBankAndFederalReserveBankStock	0	usd
CY2023	us-gaap	Proceeds From Life Insurance Policies ProceedsFromLifeInsurancePolicies	5200000	usd
CY2022	us-gaap	Proceeds From Life Insurance Policies ProceedsFromLifeInsurancePolicies	1300000	usd
CY2021	us-gaap	Proceeds From Life Insurance Policies ProceedsFromLifeInsurancePolicies	1000000.0	usd
CY2023	us-gaap	Payments For Proceeds From Other Loans And Leases PaymentsForProceedsFromOtherLoansAndLeases	185800000	usd
CY2022	us-gaap	Payments For Proceeds From Other Loans And Leases PaymentsForProceedsFromOtherLoansAndLeases	951400000	usd
CY2021	us-gaap	Payments For Proceeds From Other Loans And Leases PaymentsForProceedsFromOtherLoansAndLeases	-467800000	usd
CY2023	us-gaap	Proceeds From Sale Of Wholly Owned Real Estate And Real Estate Acquired In Settlement Of Loans ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans	3500000	usd
CY2022	us-gaap	Proceeds From Sale Of Wholly Owned Real Estate And Real Estate Acquired In Settlement Of Loans ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans	3400000	usd
CY2021	us-gaap	Proceeds From Sale Of Wholly Owned Real Estate And Real Estate Acquired In Settlement Of Loans ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans	1700000	usd
CY2023	fibk	Proceeds From The Sale Of Health Savings Accounts ProceedsFromTheSaleOfHealthSavingsAccounts	0	usd
CY2022	fibk	Proceeds From The Sale Of Health Savings Accounts ProceedsFromTheSaleOfHealthSavingsAccounts	1400000	usd
CY2021	fibk	Proceeds From The Sale Of Health Savings Accounts ProceedsFromTheSaleOfHealthSavingsAccounts	0	usd
CY2023	us-gaap	Cash Acquired From Acquisition CashAcquiredFromAcquisition	0	usd
CY2022	us-gaap	Cash Acquired From Acquisition CashAcquiredFromAcquisition	2006900000	usd
CY2021	us-gaap	Cash Acquired From Acquisition CashAcquiredFromAcquisition	0	usd
CY2023	us-gaap	Payments For Proceeds From Productive Assets PaymentsForProceedsFromProductiveAssets	28200000	usd
CY2022	us-gaap	Payments For Proceeds From Productive Assets PaymentsForProceedsFromProductiveAssets	10500000	usd
CY2021	us-gaap	Payments For Proceeds From Productive Assets PaymentsForProceedsFromProductiveAssets	10300000	usd
CY2023	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	1253400000	usd
CY2022	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-949000000.0	usd
CY2021	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-2120000000	usd
CY2023	us-gaap	Increase Decrease In Deposits IncreaseDecreaseInDeposits	-1750500000	usd
CY2022	us-gaap	Increase Decrease In Deposits IncreaseDecreaseInDeposits	-2884000000	usd
CY2021	us-gaap	Increase Decrease In Deposits IncreaseDecreaseInDeposits	2052600000	usd
CY2023	us-gaap	Proceeds From Payments For In Securities Sold Under Agreements To Repurchase ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase	-270200000	usd
CY2022	us-gaap	Proceeds From Payments For In Securities Sold Under Agreements To Repurchase ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase	28900000	usd
CY2021	us-gaap	Proceeds From Payments For In Securities Sold Under Agreements To Repurchase ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase	-40300000	usd
CY2023	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	276000000.0	usd
CY2022	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	2327000000	usd
CY2021	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	0	usd
CY2023	us-gaap	Repayments Of Other Long Term Debt RepaymentsOfOtherLongTermDebt	100000	usd
CY2022	us-gaap	Repayments Of Other Long Term Debt RepaymentsOfOtherLongTermDebt	164100000	usd
CY2021	us-gaap	Repayments Of Other Long Term Debt RepaymentsOfOtherLongTermDebt	0	usd
CY2023	us-gaap	Proceeds From Issuance Of Unsecured Debt ProceedsFromIssuanceOfUnsecuredDebt	0	usd
CY2022	us-gaap	Proceeds From Issuance Of Unsecured Debt ProceedsFromIssuanceOfUnsecuredDebt	14300000	usd
CY2021	us-gaap	Proceeds From Issuance Of Unsecured Debt ProceedsFromIssuanceOfUnsecuredDebt	0	usd
CY2023	us-gaap	Payments Of Stock Issuance Costs PaymentsOfStockIssuanceCosts	0	usd
CY2022	us-gaap	Payments Of Stock Issuance Costs PaymentsOfStockIssuanceCosts	800000	usd
CY2021	us-gaap	Payments Of Stock Issuance Costs PaymentsOfStockIssuanceCosts	0	usd
CY2023	us-gaap	Proceeds From Issuance Of Common Stock ProceedsFromIssuanceOfCommonStock	0	usd
CY2022	us-gaap	Proceeds From Issuance Of Common Stock ProceedsFromIssuanceOfCommonStock	100000	usd
CY2021	us-gaap	Proceeds From Issuance Of Common Stock ProceedsFromIssuanceOfCommonStock	400000	usd
CY2023	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	34000000.0	usd
CY2022	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	199000000.0	usd
CY2021	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	5400000	usd
CY2023	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	195100000	usd
CY2022	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	182100000	usd
CY2023	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-1973900000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	870500000	usd
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	2344800000	usd
CY2023	us-gaap	Income Taxes Paid IncomeTaxesPaid	51200000	usd
CY2022	us-gaap	Income Taxes Paid IncomeTaxesPaid	57800000	usd
CY2021	us-gaap	Income Taxes Paid IncomeTaxesPaid	56800000	usd
CY2023	us-gaap	Interest Paid Net InterestPaidNet	363600000	usd
CY2022	us-gaap	Interest Paid Net InterestPaidNet	32000000.0	usd
CY2021	us-gaap	Interest Paid Net InterestPaidNet	19300000	usd
CY2023	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	6400000	usd
CY2022	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	22900000	usd
CY2021	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	5900000	usd
CY2023	us-gaap	Proceeds From Sale Of Held To Maturity Securities ProceedsFromSaleOfHeldToMaturitySecurities	23000000.0	usd
CY2022	us-gaap	Proceeds From Sale Of Held To Maturity Securities ProceedsFromSaleOfHeldToMaturitySecurities	10900000	usd
CY2021	us-gaap	Proceeds From Sale Of Held To Maturity Securities ProceedsFromSaleOfHeldToMaturitySecurities	0	usd
CY2023	fibk	Non Cash Transfer Of Securities From Available For Sale To Held To Maturity NonCashTransferOfSecuritiesFromAvailableForSaleToHeldToMaturity	0	usd
CY2022	fibk	Non Cash Transfer Of Securities From Available For Sale To Held To Maturity NonCashTransferOfSecuritiesFromAvailableForSaleToHeldToMaturity	463600000	usd
CY2021	fibk	Non Cash Transfer Of Securities From Available For Sale To Held To Maturity NonCashTransferOfSecuritiesFromAvailableForSaleToHeldToMaturity	672200000	usd
CY2023	us-gaap	Transfer Of Loans Held For Sale To Portfolio Loans1 TransferOfLoansHeldForSaleToPortfolioLoans1	29600000	usd
CY2022	us-gaap	Transfer Of Loans Held For Sale To Portfolio Loans1 TransferOfLoansHeldForSaleToPortfolioLoans1	19800000	usd
CY2021	us-gaap	Transfer Of Loans Held For Sale To Portfolio Loans1 TransferOfLoansHeldForSaleToPortfolioLoans1	0	usd
CY2023	us-gaap	Transfer Of Portfolio Loans And Leases To Held For Sale1 TransferOfPortfolioLoansAndLeasesToHeldForSale1	30400000	usd
CY2022	us-gaap	Transfer Of Portfolio Loans And Leases To Held For Sale1 TransferOfPortfolioLoansAndLeasesToHeldForSale1	12400000	usd
CY2021	us-gaap	Transfer Of Portfolio Loans And Leases To Held For Sale1 TransferOfPortfolioLoansAndLeasesToHeldForSale1	0	usd
CY2023	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	8300000	usd
CY2022	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	400000	usd
CY2021	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	900000	usd
CY2023	us-gaap	Asset Acquisition Consideration Transferred Equity Interest Issued And Issuable AssetAcquisitionConsiderationTransferredEquityInterestIssuedAndIssuable	0	usd
CY2022	us-gaap	Asset Acquisition Consideration Transferred Equity Interest Issued And Issuable AssetAcquisitionConsiderationTransferredEquityInterestIssuedAndIssuable	1723300000	usd
CY2021	us-gaap	Asset Acquisition Consideration Transferred Equity Interest Issued And Issuable AssetAcquisitionConsiderationTransferredEquityInterestIssuedAndIssuable	0	usd
CY2023	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	1100000	usd
CY2022	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	2500000	usd
CY2021	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	3700000	usd
CY2023	us-gaap	Noncash Or Part Noncash Acquisition Investments Acquired1 NoncashOrPartNoncashAcquisitionInvestmentsAcquired1	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Investments Acquired1 NoncashOrPartNoncashAcquisitionInvestmentsAcquired1	2699000000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Investments Acquired1 NoncashOrPartNoncashAcquisitionInvestmentsAcquired1	0	usd
CY2023	us-gaap	Securities Purchased Under Agreements To Resell Increase SecuritiesPurchasedUnderAgreementsToResellIncrease	0	usd
CY2022	us-gaap	Securities Purchased Under Agreements To Resell Increase SecuritiesPurchasedUnderAgreementsToResellIncrease	101100000	usd
CY2021	us-gaap	Securities Purchased Under Agreements To Resell Increase SecuritiesPurchasedUnderAgreementsToResellIncrease	0	usd
CY2023	fibk	Noncash Or Part Noncash Acquisition Loans Held For Sale Acquired NoncashOrPartNoncashAcquisitionLoansHeldForSaleAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Loans Held For Sale Acquired NoncashOrPartNoncashAcquisitionLoansHeldForSaleAcquired	217000000.0	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Loans Held For Sale Acquired NoncashOrPartNoncashAcquisitionLoansHeldForSaleAcquired	0	usd
CY2023	fibk	Noncash Or Part Noncash Acquisition Loans Acquired NoncashOrPartNoncashAcquisitionLoansAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Loans Acquired NoncashOrPartNoncashAcquisitionLoansAcquired	7645500000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Loans Acquired NoncashOrPartNoncashAcquisitionLoansAcquired	0	usd
CY2023	us-gaap	Noncash Or Part Noncash Acquisition Fixed Assets Acquired1 NoncashOrPartNoncashAcquisitionFixedAssetsAcquired1	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Fixed Assets Acquired1 NoncashOrPartNoncashAcquisitionFixedAssetsAcquired1	144700000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Fixed Assets Acquired1 NoncashOrPartNoncashAcquisitionFixedAssetsAcquired1	0	usd
CY2023	fibk	Noncash Or Part Noncash Acquisition Goodwill NoncashOrPartNoncashAcquisitionGoodwill	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Goodwill NoncashOrPartNoncashAcquisitionGoodwill	479300000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Goodwill NoncashOrPartNoncashAcquisitionGoodwill	0	usd
CY2023	us-gaap	Noncash Or Part Noncash Acquisition Intangible Assets Acquired1 NoncashOrPartNoncashAcquisitionIntangibleAssetsAcquired1	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Intangible Assets Acquired1 NoncashOrPartNoncashAcquisitionIntangibleAssetsAcquired1	72900000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Intangible Assets Acquired1 NoncashOrPartNoncashAcquisitionIntangibleAssetsAcquired1	0	usd
CY2023	fibk	Noncash Or Part Noncash Acquisition Mortgage Servicing Asset Acquired NoncashOrPartNoncashAcquisitionMortgageServicingAssetAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Mortgage Servicing Asset Acquired NoncashOrPartNoncashAcquisitionMortgageServicingAssetAcquired	1300000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Mortgage Servicing Asset Acquired NoncashOrPartNoncashAcquisitionMortgageServicingAssetAcquired	0	usd
CY2023	fibk	Noncash Or Part Noncash Acquisition Company Owned Life Insurance Acquired NoncashOrPartNoncashAcquisitionCompanyOwnedLifeInsuranceAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Company Owned Life Insurance Acquired NoncashOrPartNoncashAcquisitionCompanyOwnedLifeInsuranceAcquired	186600000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Company Owned Life Insurance Acquired NoncashOrPartNoncashAcquisitionCompanyOwnedLifeInsuranceAcquired	0	usd
CY2023	fibk	Noncash Or Part Noncash Acquisition Deferred Tax Assets Acquired NoncashOrPartNoncashAcquisitionDeferredTaxAssetsAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Deferred Tax Assets Acquired NoncashOrPartNoncashAcquisitionDeferredTaxAssetsAcquired	60200000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Deferred Tax Assets Acquired NoncashOrPartNoncashAcquisitionDeferredTaxAssetsAcquired	0	usd
CY2023	fibk	Noncash Or Part Noncash Acquisition Other Real Estate Owned Acquired NoncashOrPartNoncashAcquisitionOtherRealEstateOwnedAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Other Real Estate Owned Acquired NoncashOrPartNoncashAcquisitionOtherRealEstateOwnedAcquired	15800000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Other Real Estate Owned Acquired NoncashOrPartNoncashAcquisitionOtherRealEstateOwnedAcquired	0	usd
CY2023	us-gaap	Noncash Or Part Noncash Acquisition Other Assets Acquired1 NoncashOrPartNoncashAcquisitionOtherAssetsAcquired1	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Other Assets Acquired1 NoncashOrPartNoncashAcquisitionOtherAssetsAcquired1	200800000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Other Assets Acquired1 NoncashOrPartNoncashAcquisitionOtherAssetsAcquired1	0	usd
CY2023	us-gaap	Noncash Or Part Noncash Acquisition Value Of Assets Acquired1 NoncashOrPartNoncashAcquisitionValueOfAssetsAcquired1	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Value Of Assets Acquired1 NoncashOrPartNoncashAcquisitionValueOfAssetsAcquired1	11824200000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Value Of Assets Acquired1 NoncashOrPartNoncashAcquisitionValueOfAssetsAcquired1	0	usd
CY2023	fibk	Noncash Or Part Noncash Acquisition Deposits Acquired NoncashOrPartNoncashAcquisitionDepositsAcquired	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Deposits Acquired NoncashOrPartNoncashAcquisitionDepositsAcquired	11688000000	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Deposits Acquired NoncashOrPartNoncashAcquisitionDepositsAcquired	0	usd
CY2023	fibk	Noncash Or Part Noncash Acquisition Securities Sold Under Repurchase Agreements NoncashOrPartNoncashAcquisitionSecuritiesSoldUnderRepurchaseAgreements	0	usd
CY2022	fibk	Noncash Or Part Noncash Acquisition Securities Sold Under Repurchase Agreements NoncashOrPartNoncashAcquisitionSecuritiesSoldUnderRepurchaseAgreements	74000000.0	usd
CY2021	fibk	Noncash Or Part Noncash Acquisition Securities Sold Under Repurchase Agreements NoncashOrPartNoncashAcquisitionSecuritiesSoldUnderRepurchaseAgreements	0	usd
CY2023	us-gaap	Noncash Or Part Noncash Acquisition Payables Assumed1 NoncashOrPartNoncashAcquisitionPayablesAssumed1	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Payables Assumed1 NoncashOrPartNoncashAcquisitionPayablesAssumed1	110400000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Payables Assumed1 NoncashOrPartNoncashAcquisitionPayablesAssumed1	0	usd
CY2023	us-gaap	Noncash Or Part Noncash Acquisition Debt Assumed1 NoncashOrPartNoncashAcquisitionDebtAssumed1	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Debt Assumed1 NoncashOrPartNoncashAcquisitionDebtAssumed1	159300000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Debt Assumed1 NoncashOrPartNoncashAcquisitionDebtAssumed1	0	usd
CY2023	fibk	Noncashor Part Noncash Acquisition Trust Preferred Securities Assumed NoncashorPartNoncashAcquisitionTrustPreferredSecuritiesAssumed	0	usd
CY2022	fibk	Noncashor Part Noncash Acquisition Trust Preferred Securities Assumed NoncashorPartNoncashAcquisitionTrustPreferredSecuritiesAssumed	76100000	usd
CY2021	fibk	Noncashor Part Noncash Acquisition Trust Preferred Securities Assumed NoncashorPartNoncashAcquisitionTrustPreferredSecuritiesAssumed	0	usd
CY2023	us-gaap	Noncash Or Part Noncash Acquisition Value Of Liabilities Assumed1 NoncashOrPartNoncashAcquisitionValueOfLiabilitiesAssumed1	0	usd
CY2022	us-gaap	Noncash Or Part Noncash Acquisition Value Of Liabilities Assumed1 NoncashOrPartNoncashAcquisitionValueOfLiabilitiesAssumed1	12107800000	usd
CY2021	us-gaap	Noncash Or Part Noncash Acquisition Value Of Liabilities Assumed1 NoncashOrPartNoncashAcquisitionValueOfLiabilitiesAssumed1	0	usd
CY2023Q4	fibk	Numberof Subsidiary Banks NumberofSubsidiaryBanks	1	subsidiary
CY2023	us-gaap	Use Of Estimates UseOfEstimates	<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:111%">Use of Estimates.</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:111%"> The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America (“GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and income and expenses during the reporting period. Actual results could differ from those estimates. Material estimates that are particularly susceptible to change relate to the determination of the allowance for credit losses, the valuation of goodwill and intangible assets, and fair values of investment securities and other financial instruments.</span></div>
CY2023Q4	fibk	Federal Reserve Bank Unrestricted Cash Held Carrying Value FederalReserveBankUnrestrictedCashHeldCarryingValue	199000000	usd
CY2022Q4	fibk	Federal Reserve Bank Unrestricted Cash Held Carrying Value FederalReserveBankUnrestrictedCashHeldCarryingValue	518900000	usd
CY2023Q4	us-gaap	Accrued Investment Income Receivable AccruedInvestmentIncomeReceivable	38500000	usd
CY2022Q4	us-gaap	Accrued Investment Income Receivable AccruedInvestmentIncomeReceivable	38900000	usd
CY2023Q4	us-gaap	Deferred Compensation Plan Assets DeferredCompensationPlanAssets	19200000	usd
CY2022Q4	us-gaap	Deferred Compensation Plan Assets DeferredCompensationPlanAssets	18700000	usd
CY2023Q4	us-gaap	Deferred Compensation Liability Current And Noncurrent DeferredCompensationLiabilityCurrentAndNoncurrent	19200000	usd
CY2022Q4	us-gaap	Deferred Compensation Liability Current And Noncurrent DeferredCompensationLiabilityCurrentAndNoncurrent	18700000	usd
CY2021	us-gaap	Income Tax Examination Penalties Expense IncomeTaxExaminationPenaltiesExpense	0	usd
CY2023	us-gaap	Income Tax Examination Penalties Expense IncomeTaxExaminationPenaltiesExpense	0	usd
CY2022	us-gaap	Impairment Of Long Lived Assets To Be Disposed Of ImpairmentOfLongLivedAssetsToBeDisposedOf	0	usd
CY2021	us-gaap	Impairment Of Long Lived Assets To Be Disposed Of ImpairmentOfLongLivedAssetsToBeDisposedOf	0	usd
CY2023	us-gaap	Impairment Of Long Lived Assets To Be Disposed Of ImpairmentOfLongLivedAssetsToBeDisposedOf	0	usd
CY2023Q4	us-gaap	Federal Reserve Bank Stock FederalReserveBankStock	94200000	usd
CY2022Q4	us-gaap	Federal Reserve Bank Stock FederalReserveBankStock	93600000	usd
CY2023Q4	us-gaap	Federal Home Loan Bank Stock FederalHomeLoanBankStock	127800000	usd
CY2022Q4	us-gaap	Federal Home Loan Bank Stock FederalHomeLoanBankStock	103800000	usd
CY2023Q4	us-gaap	Marketable Securities MarketableSecurities	1200000	usd
CY2022Q4	us-gaap	Marketable Securities MarketableSecurities	1200000	usd
CY2023	us-gaap	Derivatives Methods Of Accounting Hedging Derivatives DerivativesMethodsOfAccountingHedgingDerivatives	<div style="margin-bottom:8pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:700;line-height:119%">Derivatives and Hedging Activities. </span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:119%">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:119%">The Company formally assesses, both at the hedge's inception and on an ongoing basis, whether the derivatives that are used in hedging transactions are highly effective in offsetting changes in cash flows of hedged items. The gain or loss on cash flow hedging instruments is initially reported as a component of other comprehensive income and subsequently reclassified into earnings in the same period during which the transaction affects earnings. </span></div><div style="margin-bottom:8pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:119%">When it is determined that a derivative is not highly effective as a hedge or that it has ceased to be a highly effective hedge, the Company discontinues hedge accounting prospectively when (a) it is determined that the derivative is no longer effective in offsetting changes in the cash flows of a hedged item (including forecasted transactions); (b) the derivative expires or is sold, terminated, or exercised; (c) the derivative is de-designated as a hedge instrument, because it is unlikely that a forecasted transaction will occur; or (d) management determines that designation of the derivative as a hedge instrument is no longer appropriate.</span></div><div style="margin-bottom:8pt;padding-left:27pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:119%">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:119%">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:119%">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:119%">The Company sells residential mortgage loans on either a best efforts or mandatory delivery basis. The Company mitigates the effect of the interest rate risk inherent in providing interest rate lock commitments by entering into forward loan sales contracts. During the interest rate lock commitment period, these forward loan sales contracts are marked to market through earnings and are not designated as accounting hedges. Exclusive of the fair value component associated with the projected cash flows from the loan delivery to the investor, the changes in fair value related to movements in market rates of the interest rate lock commitments and the forward loan sales contracts generally move in opposite directions, and the net impact of changes in these valuations on net income during the loan commitment period is generally inconsequential. When the loan is funded to the borrower, the interest rate lock commitment derivative expires and the Company records a loan held for sale. The forward loan sales contract acts as a hedge against the variability in cash to be received from the loan sale. The changes in measurement of the estimated fair values of the interest rate lock commitments and forward loan sales contracts are included in mortgage banking revenues in the accompanying consolidated statements of income.</span></div>
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CY2022	us-gaap	Hosting Arrangement Service Contract Implementation Cost Expense Amortization HostingArrangementServiceContractImplementationCostExpenseAmortization	3400000	usd
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CY2022Q4	us-gaap	Goodwill Goodwill	1100900000	usd
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CY2022Q3	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	0	usd
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CY2023Q4	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Loss HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss	334300000	usd
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CY2023Q4	us-gaap	Loans And Leases Receivable Impaired Nonperforming Accrual Of Interest LoansAndLeasesReceivableImpairedNonperformingAccrualOfInterest	0	usd
CY2023	us-gaap	Impaired Financing Receivable Interest Income Accrual Method ImpairedFinancingReceivableInterestIncomeAccrualMethod	0	usd
CY2023	us-gaap	Financing Receivable Accrued Interest Writeoff FinancingReceivableAccruedInterestWriteoff	-2700000	usd
CY2022	us-gaap	Financing Receivable Accrued Interest Writeoff FinancingReceivableAccruedInterestWriteoff	1500000	usd
CY2023	us-gaap	Financing Receivable Accrued Interest Modified FinancingReceivableAccruedInterestModified	1600000	usd
CY2023Q4	us-gaap	Financing Receivable Loan In Process FinancingReceivableLoanInProcess	6700000	usd
CY2022Q1	us-gaap	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Acquired During Period Cash Flows Expected To Be Collected At Acquisition CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodCashFlowsExpectedToBeCollectedAtAcquisition	623300000	usd
CY2022Q1	us-gaap	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Allowance For Loan Losses CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses	298200000	usd
CY2022Q1	fibk	Certain Loans And Debt Securities Acquired In Transfer Discount At Acquisition CertainLoansAndDebtSecuritiesAcquiredInTransferDiscountAtAcquisition	57700000	usd
CY2022Q1	fibk	Certain Loans And Debt Securities Acquired In Transfer Part Value At Acquisition CertainLoansAndDebtSecuritiesAcquiredInTransferPartValueAtAcquisition	979200000	usd
CY2022Q1	us-gaap	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Allowance For Loan Losses CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses	298200000	usd
CY2022Q1	us-gaap	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Allowance For Loan Losses Decreases1 CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLossesDecreases1	238700000	usd
CY2022Q1	fibk	Business Combination Impact On Purchased Credit Deteriorated Loans BusinessCombinationImpactOnPurchasedCreditDeterioratedLoans	59500000	usd
CY2022Q1	fibk	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Difference Between Amortized Cost Basis And Unpaid Principal Balance CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesDifferenceBetweenAmortizedCostBasisAndUnpaidPrincipalBalance	39600000	usd
CY2022Q1	fibk	Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Interest Applied To Principal CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesInterestAppliedToPrincipal	18100000	usd
CY2022Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	45200000	usd
CY2023Q4	us-gaap	Derivative Liability Notional Amount DerivativeLiabilityNotionalAmount	2295700000	usd
CY2023Q4	us-gaap	Interest Rate Derivative Liabilities At Fair Value InterestRateDerivativeLiabilitiesAtFairValue	127300000	usd
CY2023Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	673500000	usd
CY2022Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	666900000	usd
CY2023Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	229200000	usd
CY2022Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	222200000	usd
CY2023Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	444300000	usd
CY2022Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	444700000	usd
CY2023	us-gaap	Depreciation Depreciation	34300000	usd
CY2022	us-gaap	Depreciation Depreciation	35100000	usd
CY2021	us-gaap	Depreciation Depreciation	28000000	usd
CY2023Q4	us-gaap	Life Insurance Corporate Or Bank Owned Amount LifeInsuranceCorporateOrBankOwnedAmount	502400000	usd
CY2022Q4	us-gaap	Life Insurance Corporate Or Bank Owned Amount LifeInsuranceCorporateOrBankOwnedAmount	497900000	usd
CY2022Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	12700000	usd
CY2021Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	2000000.0	usd
CY2020Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	2500000	usd
CY2023	fibk	Real Estate Acquired Through Foreclosure Acquired Through Acquisition RealEstateAcquiredThroughForeclosureAcquiredThroughAcquisition	0	usd
CY2022	fibk	Real Estate Acquired Through Foreclosure Acquired Through Acquisition RealEstateAcquiredThroughForeclosureAcquiredThroughAcquisition	15800000	usd
CY2021	fibk	Real Estate Acquired Through Foreclosure Acquired Through Acquisition RealEstateAcquiredThroughForeclosureAcquiredThroughAcquisition	0	usd
CY2023	fibk	Real Estate Acquired Through Foreclosure Additions RealEstateAcquiredThroughForeclosureAdditions	8300000	usd
CY2022	fibk	Real Estate Acquired Through Foreclosure Additions RealEstateAcquiredThroughForeclosureAdditions	400000	usd
CY2021	fibk	Real Estate Acquired Through Foreclosure Additions RealEstateAcquiredThroughForeclosureAdditions	900000	usd
CY2023	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	1100000	usd
CY2022	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	2800000	usd
CY2021	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	0	usd
CY2023	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	3400000	usd
CY2022	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	2700000	usd
CY2021	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	1400000	usd
CY2023Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	16500000	usd
CY2022Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	12700000	usd
CY2021Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	2000000.0	usd
CY2023	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	1100000	usd
CY2022	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	2800000	usd
CY2023Q4	fibk	Foreclosed Residential Real Estateincludedin Other Real Estate Owned ForeclosedResidentialRealEstateincludedinOtherRealEstateOwned	0	usd
CY2023Q4	us-gaap	Mortgage Loans In Process Of Foreclosure Amount MortgageLoansInProcessOfForeclosureAmount	500000	usd
CY2023Q4	us-gaap	Derivative Instruments And Hedges Noncurrent DerivativeInstrumentsAndHedgesNoncurrent	900000000	usd
CY2023Q4	us-gaap	Interest Rate Cash Flow Hedge Gain Loss To Be Reclassified During Next12 Months Net InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet	9100000	usd
CY2023	us-gaap	Interest Income Hedge Basis Adjustment Portfolio Layer Method InterestIncomeHedgeBasisAdjustmentPortfolioLayerMethod	1400000	usd
CY2023Q4	us-gaap	Derivative Instruments And Hedges DerivativeInstrumentsAndHedges	193300000	usd
CY2023Q4	us-gaap	Hedged Asset Fair Value Hedge Cumulative Increase Decrease HedgedAssetFairValueHedgeCumulativeIncreaseDecrease	6700000	usd
CY2022Q4	us-gaap	Derivative Instruments And Hedges DerivativeInstrumentsAndHedges	191900000	usd
CY2022Q4	us-gaap	Hedged Asset Fair Value Hedge Cumulative Increase Decrease HedgedAssetFairValueHedgeCumulativeIncreaseDecrease	8100000	usd
CY2023Q4	us-gaap	Derivative Asset Notional Amount DerivativeAssetNotionalAmount	2144700000	usd
CY2023Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	37400000	usd
CY2022Q4	us-gaap	Derivative Asset Notional Amount DerivativeAssetNotionalAmount	2290700000	usd
CY2022Q4	us-gaap	Derivative Liability Notional Amount DerivativeLiabilityNotionalAmount	2449000000	usd
CY2022Q4	us-gaap	Interest Rate Derivative Liabilities At Fair Value InterestRateDerivativeLiabilitiesAtFairValue	159600000	usd
CY2023	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationTax	6500000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationTax	2200000	usd
CY2023	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Before Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax	-7500000	usd
CY2023Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	37400000	usd
CY2023Q4	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	usd
CY2023Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	37400000	usd
CY2023Q4	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	usd
CY2023Q4	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	32700000	usd
CY2023Q4	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	4700000	usd
CY2023Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	37400000	usd
CY2023Q4	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	usd
CY2023Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	37400000	usd
CY2023Q4	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	usd
CY2023Q4	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	32700000	usd
CY2023Q4	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	4700000	usd
CY2023Q4	us-gaap	Derivative Fair Value Of Derivative Liability DerivativeFairValueOfDerivativeLiability	127300000	usd
CY2023Q4	us-gaap	Derivative Liability Fair Value Gross Asset DerivativeLiabilityFairValueGrossAsset	0	usd
CY2023Q4	us-gaap	Interest Rate Derivative Liabilities At Fair Value InterestRateDerivativeLiabilitiesAtFairValue	127300000	usd
CY2023Q4	us-gaap	Derivative Collateral Right To Reclaim Securities DerivativeCollateralRightToReclaimSecurities	0	usd
CY2023Q4	us-gaap	Derivative Collateral Right To Reclaim Cash DerivativeCollateralRightToReclaimCash	0	usd
CY2023Q4	us-gaap	Derivative Fair Value Of Derivative Liability Amount Offset Against Collateral DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral	127300000	usd
CY2023Q4	us-gaap	Securities Sold Under Agreements To Repurchase Gross SecuritiesSoldUnderAgreementsToRepurchaseGross	782700000	usd
CY2023Q4	us-gaap	Securities Sold Under Agreements To Repurchase Asset SecuritiesSoldUnderAgreementsToRepurchaseAsset	0	usd
CY2023Q4	us-gaap	Securities Sold Under Agreements To Repurchase SecuritiesSoldUnderAgreementsToRepurchase	782700000	usd
CY2023Q4	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Securities SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimSecurities	0	usd
CY2023Q4	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Cash SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimCash	782700000	usd
CY2023Q4	us-gaap	Securities Sold Under Agreements To Repurchase Amount Offset Against Collateral SecuritiesSoldUnderAgreementsToRepurchaseAmountOffsetAgainstCollateral	0	usd
CY2023Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Gross DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedGross	910000000.0	usd
CY2023Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Asset DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset	0	usd
CY2023Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoaned	910000000.0	usd
CY2023Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Securities DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimSecurities	0	usd
CY2023Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Cash DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimCash	782700000	usd
CY2023Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Amount Offset Against Collateral DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAmountOffsetAgainstCollateral	127300000	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	45200000	usd
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CY2022Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	45200000	usd
CY2022Q4	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	usd
CY2022Q4	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	45100000	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	100000	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	45200000	usd
CY2022Q4	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	usd
CY2022Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	45200000	usd
CY2022Q4	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	usd
CY2022Q4	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	45100000	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	100000	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Liability DerivativeFairValueOfDerivativeLiability	159600000	usd
CY2022Q4	us-gaap	Derivative Liability Fair Value Gross Asset DerivativeLiabilityFairValueGrossAsset	0	usd
CY2022Q4	us-gaap	Interest Rate Derivative Liabilities At Fair Value InterestRateDerivativeLiabilitiesAtFairValue	159600000	usd
CY2022Q4	us-gaap	Derivative Collateral Right To Reclaim Securities DerivativeCollateralRightToReclaimSecurities	0	usd
CY2022Q4	us-gaap	Derivative Collateral Right To Reclaim Cash DerivativeCollateralRightToReclaimCash	0	usd
CY2022Q4	us-gaap	Derivative Fair Value Of Derivative Liability Amount Offset Against Collateral DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral	159600000	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase Gross SecuritiesSoldUnderAgreementsToRepurchaseGross	1052900000	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase Asset SecuritiesSoldUnderAgreementsToRepurchaseAsset	0	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase SecuritiesSoldUnderAgreementsToRepurchase	1052900000	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Securities SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimSecurities	0	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Cash SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimCash	1052900000	usd
CY2022Q4	us-gaap	Securities Sold Under Agreements To Repurchase Amount Offset Against Collateral SecuritiesSoldUnderAgreementsToRepurchaseAmountOffsetAgainstCollateral	0	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Gross DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedGross	1212500000	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Asset DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset	0	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoaned	1212500000	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Securities DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimSecurities	0	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Cash DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimCash	1052900000	usd
CY2022Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Amount Offset Against Collateral DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAmountOffsetAgainstCollateral	159600000	usd
CY2023Q4	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	6029600000	usd
CY2022Q4	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	7560000000	usd
CY2023Q4	us-gaap	Demand Deposit Accounts DemandDepositAccounts	6507800000	usd
CY2022Q4	us-gaap	Demand Deposit Accounts DemandDepositAccounts	7205900000	usd
CY2023Q4	us-gaap	Deposits Savings Deposits DepositsSavingsDeposits	7775800000	usd
CY2022Q4	us-gaap	Deposits Savings Deposits DepositsSavingsDeposits	8379300000	usd
CY2023Q4	us-gaap	Time Deposits100000 Or More TimeDeposits100000OrMore	811600000	usd
CY2022Q4	us-gaap	Time Deposits100000 Or More TimeDeposits100000OrMore	438000000.0	usd
CY2023Q4	us-gaap	Time Deposits Less Than100000 TimeDepositsLessThan100000	2198300000	usd
CY2022Q4	us-gaap	Time Deposits Less Than100000 TimeDepositsLessThan100000	1490400000	usd
CY2023Q4	us-gaap	Interest Bearing Deposit Liabilities InterestBearingDepositLiabilities	17293500000	usd
CY2022Q4	us-gaap	Interest Bearing Deposit Liabilities InterestBearingDepositLiabilities	17513600000	usd
CY2023Q4	us-gaap	Deposits Deposits	23323100000	usd
CY2022Q4	us-gaap	Deposits Deposits	25073600000	usd
CY2023Q4	fibk	Fdic Deposit Insurance Limit FDICDepositInsuranceLimit	250000000	usd
CY2023Q4	fibk	Time Deposits Obtained Certificate Of Deposit Account Registry Service TimeDepositsObtainedCertificateOfDepositAccountRegistryService	26600000	usd
CY2022Q4	fibk	Time Deposits Obtained Certificate Of Deposit Account Registry Service TimeDepositsObtainedCertificateOfDepositAccountRegistryService	36600000	usd
CY2023Q4	us-gaap	Interest Bearing Domestic Deposit Brokered InterestBearingDomesticDepositBrokered	0	usd
CY2023Q4	fibk	Time Deposits250000or More TimeDeposits250000orMore	811600000	usd
CY2022Q4	fibk	Time Deposits250000or More TimeDeposits250000orMore	438000000	usd
CY2023Q4	fibk	Fdic Deposit Insurance Limit FDICDepositInsuranceLimit	250000000	usd
CY2023Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More Three Months Or Less ContractualMaturitiesTimeDeposits100000OrMoreThreeMonthsOrLess	219700000	usd
CY2023Q4	fibk	Contractual Maturities Time Deposits Three Monthsor Less ContractualMaturitiesTimeDepositsThreeMonthsorLess	770400000	usd
CY2023Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More Three Months Through Six Months ContractualMaturitiesTimeDeposits100000OrMoreThreeMonthsThroughSixMonths	233900000	usd
CY2023Q4	fibk	Contractual Maturities Time Deposits Three Monthsthrough Six Months ContractualMaturitiesTimeDepositsThreeMonthsthroughSixMonths	854000000.0	usd
CY2023Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More Six Months Through12 Months ContractualMaturitiesTimeDeposits100000OrMoreSixMonthsThrough12Months	279500000	usd
CY2023Q4	fibk	Contractual Maturities Time Deposits Six Monthsthrough Twelve Months ContractualMaturitiesTimeDepositsSixMonthsthroughTwelveMonths	1024300000	usd
CY2023Q4	us-gaap	Contractual Maturities Time Deposits100000 Or More After12 Months ContractualMaturitiesTimeDeposits100000OrMoreAfter12Months	62800000	usd
CY2023Q4	us-gaap	Time Deposits Noncurrent TimeDepositsNoncurrent	277100000	usd
CY2023Q4	fibk	Contractual Maturities Time Deposits100000or Moreafter2years ContractualMaturitiesTimeDeposits100000orMoreafter2years	9100000	usd
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CY2023Q4	us-gaap	Time Deposits100000 Or More TimeDeposits100000OrMore	811600000	usd
CY2023Q4	us-gaap	Time Deposits TimeDeposits	3009900000	usd
CY2023	us-gaap	Interest Expense Time Deposits100000 Or More InterestExpenseTimeDeposits100000OrMore	21500000	usd
CY2022	us-gaap	Interest Expense Time Deposits100000 Or More InterestExpenseTimeDeposits100000OrMore	2500000	usd
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CY2023Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	120800000	usd
CY2022Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	120800000	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	100000	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	100000	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	100000	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Four LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour	200000	usd
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CY2023Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	120800000	usd
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CY2023Q4	us-gaap	Advances From Federal Home Loan Banks AdvancesFromFederalHomeLoanBanks	2603000000	usd
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CY2023Q4	us-gaap	Federal Home Loan Bank Advances FederalHomeLoanBankAdvances	0	usd
CY2023Q4	us-gaap	Other Borrowings OtherBorrowings	0	usd
CY2022Q4	us-gaap	Advances From Federal Home Loan Banks AdvancesFromFederalHomeLoanBanks	2327000000	usd
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CY2023Q4	us-gaap	Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust	163100000	usd
CY2023Q4	us-gaap	Subordinated Debt SubordinatedDebt	163700000	usd
CY2023Q4	fibk	Common Stock Held In Assets CommonStockHeldInAssets	5300000	usd
CY2022Q4	us-gaap	Subordinated Debt SubordinatedDebt	163700000	usd
CY2022Q4	fibk	Common Stock Held In Assets CommonStockHeldInAssets	5300000	usd
CY2023Q4	fibk	Subordinated Debt Fair Value Adjustment SubordinatedDebtFairValueAdjustment	600000	usd
CY2022Q4	fibk	Subordinated Debt Fair Value Adjustment SubordinatedDebtFairValueAdjustment	600000	usd
CY2023Q4	us-gaap	Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust	163100000	usd
CY2022Q4	us-gaap	Subordinated Long Term Debt SubordinatedLongTermDebt	163100000	usd
CY2022Q1	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	250000000	shares
CY2023Q4	fibk	Capital Stock Authorized CapitalStockAuthorized	150100000	shares
CY2023Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	150000000	shares
CY2023Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	100000	shares
CY2023Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	103941626	shares
CY2023Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	103941626	shares
CY2022Q4	us-gaap	Preferred Stock No Par Value PreferredStockNoParValue	0
CY2023Q4	us-gaap	Preferred Stock No Par Value PreferredStockNoParValue	0
CY2023Q4	us-gaap	Treasury Stock Acquired Average Cost Per Share TreasuryStockAcquiredAverageCostPerShare	32.14
CY2023Q4	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	32100000	usd
CY2021Q4	fibk	Sale Of Stock Shares Authorized For Future Sale SaleOfStockSharesAuthorizedForFutureSale	47158390	shares
CY2023Q2	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Authorized ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized	2000000000000	shares
CY2023	us-gaap	Net Income Loss NetIncomeLoss	257500000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	202200000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	192100000	usd
CY2023	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	103752206	shares
CY2022	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	103274070	shares
CY2021	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	61650312	shares
CY2023	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	27897	shares
CY2022	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	66929	shares
CY2021	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	91516	shares
CY2023	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	103780103	shares
CY2022	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	103340999	shares
CY2021	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	61741828	shares
CY2023	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	2.48
CY2022	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.96
CY2021	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	3.12
CY2023	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	2.48
CY2022	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.96
CY2021	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	3.11
CY2023Q4	us-gaap	Capital Capital	2941100000	usd
CY2023Q4	us-gaap	Capital To Risk Weighted Assets CapitalToRiskWeightedAssets	0.1328
CY2023Q4	us-gaap	Capital Required For Capital Adequacy CapitalRequiredForCapitalAdequacy	1771600000	usd
CY2023Q4	us-gaap	Capital Required For Capital Adequacy To Risk Weighted Assets CapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0800
CY2023Q4	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	2325200000	usd
CY2023Q4	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.1050
CY2023Q4	us-gaap	Capital Required To Be Well Capitalized CapitalRequiredToBeWellCapitalized	2214500000	usd
CY2023Q4	us-gaap	Capital Required To Be Well Capitalized To Risk Weighted Assets CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.1000
CY2023Q4	us-gaap	Tier One Risk Based Capital TierOneRiskBasedCapital	2454400000	usd
CY2023Q4	us-gaap	Tier One Risk Based Capital To Risk Weighted Assets TierOneRiskBasedCapitalToRiskWeightedAssets	0.1108
CY2023Q4	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy TierOneRiskBasedCapitalRequiredForCapitalAdequacy	1328700000	usd
CY2023Q4	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0600
CY2023Q4	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	1882300000	usd
CY2023Q4	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0850
CY2023Q4	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized TierOneRiskBasedCapitalRequiredToBeWellCapitalized	1771600000	usd
CY2023Q4	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.0800
CY2023Q4	fibk	Common Equity Tier One Risked Based Capital CommonEquityTierOneRiskedBasedCapital	2454400000	usd
CY2023Q4	fibk	Common Equity Tier One Risk Based Capital To Risk Weighted Assets CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets	0.1108
CY2023Q4	fibk	Common Equity Tier One Capital Required For Capital Adequacy Under Basel Iii Phase In Schedule CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInSchedule	996500000	usd
CY2023Q4	fibk	Common Equity Tier One Capital Required For Capital Adequacy Under Basel Iii Phase In Schedule To Risk Weighted Assets CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInScheduleToRiskWeightedAssets	0.0450
CY2023Q4	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	1550100000	usd
CY2023Q4	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0700
CY2023Q4	fibk	Common Equity Tier One Capital To Be Well Capitalized CommonEquityTierOneCapitalToBeWellCapitalized	1439400000	usd
CY2023Q4	fibk	Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets CommonEquityTierOneCapitalToBeWellCapitalizedToRiskWeightedAssets	0.0650
CY2023Q4	us-gaap	Tier One Leverage Capital TierOneLeverageCapital	2454400000	usd
CY2023Q4	us-gaap	Tier One Leverage Capital To Average Assets TierOneLeverageCapitalToAverageAssets	0.0822
CY2023Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	1193900000	usd
CY2023Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2023Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	1193900000	usd
CY2023Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2023Q4	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized TierOneLeverageCapitalRequiredToBeWellCapitalized	1492300000	usd
CY2023Q4	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized To Average Assets TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets	0.0500
CY2022Q4	us-gaap	Capital Capital	2875800000	usd
CY2022Q4	us-gaap	Capital To Risk Weighted Assets CapitalToRiskWeightedAssets	0.1248
CY2022Q4	us-gaap	Capital Required For Capital Adequacy CapitalRequiredForCapitalAdequacy	1843200000	usd
CY2022Q4	us-gaap	Capital Required For Capital Adequacy To Risk Weighted Assets CapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0800
CY2022Q4	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	2419200000	usd
CY2022Q4	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.1050
CY2022Q4	us-gaap	Capital Required To Be Well Capitalized CapitalRequiredToBeWellCapitalized	2304000000	usd
CY2022Q4	us-gaap	Capital Required To Be Well Capitalized To Risk Weighted Assets CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.1000
CY2022Q4	us-gaap	Tier One Risk Based Capital TierOneRiskBasedCapital	2408800000	usd
CY2022Q4	us-gaap	Tier One Risk Based Capital To Risk Weighted Assets TierOneRiskBasedCapitalToRiskWeightedAssets	0.1045
CY2022Q4	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy TierOneRiskBasedCapitalRequiredForCapitalAdequacy	1382400000	usd
CY2022Q4	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0600
CY2022Q4	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	1958400000	usd
CY2022Q4	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0850
CY2022Q4	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized TierOneRiskBasedCapitalRequiredToBeWellCapitalized	1843200000	usd
CY2022Q4	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.0800
CY2022Q4	fibk	Common Equity Tier One Risked Based Capital CommonEquityTierOneRiskedBasedCapital	2408800000	usd
CY2022Q4	fibk	Common Equity Tier One Risk Based Capital To Risk Weighted Assets CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets	0.1045
CY2022Q4	fibk	Common Equity Tier One Capital Required For Capital Adequacy Under Basel Iii Phase In Schedule CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInSchedule	1036800000	usd
CY2022Q4	fibk	Common Equity Tier One Capital Required For Capital Adequacy Under Basel Iii Phase In Schedule To Risk Weighted Assets CommonEquityTierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIPhaseInScheduleToRiskWeightedAssets	0.0450
CY2022Q4	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	1612800000	usd
CY2022Q4	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0700
CY2022Q4	fibk	Common Equity Tier One Capital To Be Well Capitalized CommonEquityTierOneCapitalToBeWellCapitalized	1497600000	usd
CY2022Q4	fibk	Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets CommonEquityTierOneCapitalToBeWellCapitalizedToRiskWeightedAssets	0.0650
CY2022Q4	us-gaap	Tier One Leverage Capital TierOneLeverageCapital	2408800000	usd
CY2022Q4	us-gaap	Tier One Leverage Capital To Average Assets TierOneLeverageCapitalToAverageAssets	0.0775
CY2022Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	1242900000	usd
CY2022Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2022Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	1242900000	usd
CY2022Q4	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2022Q4	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized TierOneLeverageCapitalRequiredToBeWellCapitalized	1553600000	usd
CY2022Q4	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized To Average Assets TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets	0.0500
CY2020Q1	us-gaap	Income Tax Effects Allocated Directly To Equity Cumulative Effect Of Change In Accounting Principle IncomeTaxEffectsAllocatedDirectlyToEquityCumulativeEffectOfChangeInAccountingPrinciple	24100000	usd
CY2023	us-gaap	Operating Lease Expense OperatingLeaseExpense	9300000	usd
CY2022	us-gaap	Operating Lease Expense OperatingLeaseExpense	8900000	usd
CY2021	us-gaap	Lease And Rental Expense LeaseAndRentalExpense	4300000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	50600000	usd
CY2022Q4	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	16200000	usd
CY2021Q4	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	3800000	usd
CY2023Q4	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	18400000	usd
CY2022Q4	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	16200000	usd
CY2023	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	46100000	usd
CY2022	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	45200000	usd
CY2021	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	39400000	usd
CY2023	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	12800000	usd
CY2022	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	14400000	usd
CY2021	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	11300000	usd
CY2023	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	58900000	usd
CY2022	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	59600000	usd
CY2021	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	50700000	usd
CY2023	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	16000000.0	usd
CY2022	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-3700000	usd
CY2021	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	3700000	usd
CY2023	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	4400000	usd
CY2022	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-1000000.0	usd
CY2021	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	1300000	usd
CY2023	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	20400000	usd
CY2022	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-4700000	usd
CY2021	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	5000000.0	usd
CY2023	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	79300000	usd
CY2022	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	54900000	usd
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	55700000	usd
CY2023	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	70700000	usd
CY2022	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	54000000.0	usd
CY2021	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	52000000.0	usd
CY2023	us-gaap	Income Tax Reconciliation Tax Exempt Income IncomeTaxReconciliationTaxExemptIncome	8100000	usd
CY2022	us-gaap	Income Tax Reconciliation Tax Exempt Income IncomeTaxReconciliationTaxExemptIncome	8400000	usd
CY2021	us-gaap	Income Tax Reconciliation Tax Exempt Income IncomeTaxReconciliationTaxExemptIncome	2800000	usd
CY2023	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	13600000	usd
CY2022	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	10600000	usd
CY2021	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	9900000	usd
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Share Based Compensation Excess Tax Benefit Amount EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitAmount	300000	usd
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Share Based Compensation Excess Tax Benefit Amount EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitAmount	200000	usd
CY2021	us-gaap	Effective Income Tax Rate Reconciliation Share Based Compensation Excess Tax Benefit Amount EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitAmount	-500000	usd
CY2023	us-gaap	Income Tax Reconciliation Nondeductible Expense IncomeTaxReconciliationNondeductibleExpense	0	usd
CY2022	us-gaap	Income Tax Reconciliation Nondeductible Expense IncomeTaxReconciliationNondeductibleExpense	2000000.0	usd
CY2021	us-gaap	Income Tax Reconciliation Nondeductible Expense IncomeTaxReconciliationNondeductibleExpense	800000	usd
CY2023	us-gaap	Income Tax Credits And Adjustments IncomeTaxCreditsAndAdjustments	-200000	usd
CY2022	us-gaap	Income Tax Credits And Adjustments IncomeTaxCreditsAndAdjustments	-4300000	usd
CY2021	us-gaap	Income Tax Credits And Adjustments IncomeTaxCreditsAndAdjustments	-4300000	usd
CY2023	fibk	Effective Income Tax Rate Reconciliation Fdic Premiums Amount EffectiveIncomeTaxRateReconciliationFDICPremiumsAmount	2000000.0	usd
CY2022	fibk	Effective Income Tax Rate Reconciliation Fdic Premiums Amount EffectiveIncomeTaxRateReconciliationFDICPremiumsAmount	1600000	usd
CY2021	fibk	Effective Income Tax Rate Reconciliation Fdic Premiums Amount EffectiveIncomeTaxRateReconciliationFDICPremiumsAmount	0	usd
CY2023	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	1000000.0	usd
CY2022	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	-800000	usd
CY2021	us-gaap	Income Tax Reconciliation Other Adjustments IncomeTaxReconciliationOtherAdjustments	600000	usd
CY2023	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	79300000	usd
CY2022	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	54900000	usd
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	55700000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Provision For Loan Losses DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsProvisionForLoanLosses	56700000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Provision For Loan Losses DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsProvisionForLoanLosses	54800000	usd
CY2023Q4	fibk	Deferredtaxassetloandiscount Deferredtaxassetloandiscount	21700000	usd
CY2022Q4	fibk	Deferredtaxassetloandiscount Deferredtaxassetloandiscount	32500000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Unrealized Losses On Availablefor Sale Securities Gross DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross	118100000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Unrealized Losses On Availablefor Sale Securities Gross DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross	157900000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Derivative Instruments DeferredTaxAssetsDerivativeInstruments	300000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Derivative Instruments DeferredTaxAssetsDerivativeInstruments	500000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Employee Benefits DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits	18600000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Employee Benefits DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits	26800000	usd
CY2023Q4	fibk	Deferred Tax Assets Non Performing Loan Interest DeferredTaxAssetsNonPerformingLoanInterest	2200000	usd
CY2022Q4	fibk	Deferred Tax Assets Non Performing Loan Interest DeferredTaxAssetsNonPerformingLoanInterest	2600000	usd
CY2023Q4	fibk	Deferred Tax Assets Other Real Estate Owned Write Downs And Carrying Costs DeferredTaxAssetsOtherRealEstateOwnedWriteDownsAndCarryingCosts	1400000	usd
CY2022Q4	fibk	Deferred Tax Assets Other Real Estate Owned Write Downs And Carrying Costs DeferredTaxAssetsOtherRealEstateOwnedWriteDownsAndCarryingCosts	1500000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	1100000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	1500000	usd
CY2023Q4	fibk	Deferred Tax Liability Lease Liabilities DeferredTaxLiabilityLeaseLiabilities	11000000.0	usd
CY2022Q4	fibk	Deferred Tax Liability Lease Liabilities DeferredTaxLiabilityLeaseLiabilities	12000000.0	usd
CY2023Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Reserves DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsReserves	10700000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Reserves DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsReserves	8800000	usd
CY2023Q4	fibk	Deferred Tax Assets Contract Incentives DeferredTaxAssetsContractIncentives	7500000	usd
CY2022Q4	fibk	Deferred Tax Assets Contract Incentives DeferredTaxAssetsContractIncentives	8500000	usd
CY2023Q4	fibk	Deferred Tax Assets Discount On Acquired Investment Securities DeferredTaxAssetsDiscountOnAcquiredInvestmentSecurities	10100000	usd
CY2022Q4	fibk	Deferred Tax Assets Discount On Acquired Investment Securities DeferredTaxAssetsDiscountOnAcquiredInvestmentSecurities	13400000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	4800000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	2400000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	264200000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	323200000	usd
CY2023Q4	us-gaap	Deferred Tax Liabilities Property Plant And Equipment DeferredTaxLiabilitiesPropertyPlantAndEquipment	19800000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Property Plant And Equipment DeferredTaxLiabilitiesPropertyPlantAndEquipment	17300000	usd
CY2023Q4	fibk	Deferred Tax Liabilities Deferred Loan Costs DeferredTaxLiabilitiesDeferredLoanCosts	4100000	usd
CY2022Q4	fibk	Deferred Tax Liabilities Deferred Loan Costs DeferredTaxLiabilitiesDeferredLoanCosts	3300000	usd
CY2023Q4	us-gaap	Deferred Tax Liabilities Investment In Noncontrolled Affiliates DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates	2400000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Investment In Noncontrolled Affiliates DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates	1300000	usd
CY2023Q4	fibk	Deferred Tax Assets Rightof Use Asset DeferredTaxAssetsRightofUseAsset	10300000	usd
CY2022Q4	fibk	Deferred Tax Assets Rightof Use Asset DeferredTaxAssetsRightofUseAsset	11300000	usd
CY2023Q4	us-gaap	Deferred Tax Liabilities Prepaid Expenses DeferredTaxLiabilitiesPrepaidExpenses	900000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Prepaid Expenses DeferredTaxLiabilitiesPrepaidExpenses	900000	usd
CY2023Q4	fibk	Deferred Tax Liabilities Agency Stock Dividends DeferredTaxLiabilitiesAgencyStockDividends	1300000	usd
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$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

2024 Q1 Form 10-K Financial Statement

#000086041324000016 Filed on February 29, 2024

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$FIBK FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

2024 Q1 Form 10-K Financial Statement

#000086041324000016 Filed on February 29, 2024

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk