$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

$fibk Overview Financials Valuation SEC Filings Company Website

2024 Q3 Form 10-Q Financial Statement

#000086041324000130 Filed on November 04, 2024

Income Statement

Concept	2024 Q3
Revenue	$328.0M
YoY Change	1.67%
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
YoY Change
% of Gross Profit
Operating Expenses
YoY Change
Operating Profit
YoY Change
Interest Expense	$122.5M
YoY Change	12.49%
% of Operating Profit
Other Income/Expense, Net
YoY Change
Pretax Income	$72.70M
YoY Change	-23.23%
Income Tax	$17.20M
% Of Pretax Income	23.66%
Net Earnings	$55.50M
YoY Change	-23.66%
Net Earnings / Revenue	16.92%
Basic Earnings Per Share	$0.54
Diluted Earnings Per Share	$0.54
COMMON SHARES
Basic Shares Outstanding	104.5M shares
Diluted Shares Outstanding	103.2M shares

Balance Sheet

Concept	2024 Q3
SHORT-TERM ASSETS
Cash & Short-Term Investments
YoY Change
Cash & Equivalents	$698.6M
Short-Term Investments
Other Short-Term Assets
YoY Change
Inventory
Prepaid Expenses
Receivables
Other Receivables
Total Short-Term Assets
YoY Change
LONG-TERM ASSETS
Property, Plant & Equipment	$432.7M
YoY Change	-3.05%
Goodwill	$1.101B
YoY Change	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	$29.60B
YoY Change	-3.1%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable
YoY Change
Accrued Expenses
YoY Change
Deferred Revenue
YoY Change
Short-Term Debt
YoY Change
Long-Term Debt Due
YoY Change
Total Short-Term Liabilities
YoY Change
LONG-TERM LIABILITIES
Long-Term Debt
YoY Change
Other Long-Term Liabilities
YoY Change
Total Long-Term Liabilities
YoY Change
TOTAL LIABILITIES
Total Short-Term Liabilities
Total Long-Term Liabilities
Total Liabilities	$26.23B
YoY Change	-4.46%
SHAREHOLDERS EQUITY
Retained Earnings	$1.163B
YoY Change	3.65%
Common Stock
YoY Change
Preferred Stock
YoY Change
Treasury Stock (at cost)
YoY Change
Treasury Stock Shares
Shareholders Equity	$3.366B
YoY Change
Total Liabilities & Shareholders Equity	$29.60B
YoY Change	-3.1%

Cashflow Statement

Concept	2024 Q3
OPERATING ACTIVITIES
Net Income	$55.50M
YoY Change	-23.66%
Depreciation, Depletion And Amortization
YoY Change
Cash From Operating Activities
YoY Change
INVESTING ACTIVITIES
Capital Expenditures
YoY Change
Acquisitions
YoY Change
Other Investing Activities
YoY Change
Cash From Investing Activities
YoY Change
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net
YoY Change
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities
YoY Change
NET CHANGE
Cash From Operating Activities
Cash From Investing Activities
Cash From Financing Activities
Net Change In Cash
YoY Change
FREE CASH FLOW
Cash From Operating Activities
Capital Expenditures
Free Cash Flow
YoY Change

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
	dei	Amendment Flag AmendmentFlag	false
	dei	Entity Central Index Key EntityCentralIndexKey	0000860413
	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2024
	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	Q3
	dei	Document Quarterly Report DocumentQuarterlyReport	true
	dei	Document Period End Date DocumentPeriodEndDate	2024-09-30
	dei	Document Transition Report DocumentTransitionReport	false
	dei	Entity Registrant Name EntityRegistrantName	FIRST INTERSTATE BANCSYSTEM, INC.
	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	DE
	dei	Entity Small Business EntitySmallBusiness	false
	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
CY2024Q3	us-gaap	Net Income Loss NetIncomeLoss	55500000	usd
CY2023Q3	us-gaap	Net Income Loss NetIncomeLoss	72700000	usd
	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
	dei	Entity Filer Category EntityFilerCategory	Large Accelerated Filer
CY2023Q4	us-gaap	Other Long Term Debt OtherLongTermDebt	120800000	usd
	dei	Local Phone Number LocalPhoneNumber	255-5311
	dei	Security12b Title Security12bTitle	Common Stock, $0.00001 par value
	dei	Trading Symbol TradingSymbol	FIBK
	dei	Security Exchange Name SecurityExchangeName	NASDAQ
CY2024Q3	us-gaap	Occupancy Net OccupancyNet	11800000	usd
	us-gaap	Noninterest Income Other Operating Income NoninterestIncomeOtherOperatingIncome	10600000	usd
	us-gaap	Noninterest Income NoninterestIncome	131100000	usd
	us-gaap	Noninterest Income NoninterestIncome	102500000	usd
	us-gaap	Labor And Related Expense LaborAndRelatedExpense	199100000	usd
CY2024Q3	us-gaap	Other Labor Related Expenses OtherLaborRelatedExpenses	19700000	usd
CY2023Q3	us-gaap	Other Labor Related Expenses OtherLaborRelatedExpenses	19700000	usd
	us-gaap	Other Labor Related Expenses OtherLaborRelatedExpenses	55900000	usd
CY2024Q3	us-gaap	Information Technology And Data Processing InformationTechnologyAndDataProcessing	14600000	usd
CY2023Q3	us-gaap	Information Technology And Data Processing InformationTechnologyAndDataProcessing	14500000	usd
CY2024Q3	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	0	usd
	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-1339200000	usd
	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	120600000	usd
	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	-277400000	usd
CY2024Q3	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	698600000	usd
CY2023Q3	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	593100000	usd
	us-gaap	Interest Income Securities Taxable InterestIncomeSecuritiesTaxable	187500000	usd
	us-gaap	Interest Income Securities Taxable InterestIncomeSecuritiesTaxable	204300000	usd
CY2024Q3	us-gaap	Interest Income Securities Tax Exempt InterestIncomeSecuritiesTaxExempt	700000	usd
CY2023Q3	us-gaap	Interest Income Securities Tax Exempt InterestIncomeSecuritiesTaxExempt	800000	usd
	dei	Entity Address Address Line1 EntityAddressAddressLine1	401 North 31st Street
	dei	Entity Address City Or Town EntityAddressCityOrTown	Billings,
	dei	Entity Address State Or Province EntityAddressStateOrProvince	MT
	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	59101
	dei	City Area Code CityAreaCode	406
	dei	Entity Shell Company EntityShellCompany	false
CY2024Q3	dei	Entity Common Stock Shares Outstanding EntityCommonStockSharesOutstanding	104529863	shares
CY2024Q3	us-gaap	Cash And Due From Banks CashAndDueFromBanks	438900000	usd
CY2023Q4	us-gaap	Cash And Due From Banks CashAndDueFromBanks	378200000	usd
CY2024Q3	us-gaap	Interest Bearing Deposits In Banks InterestBearingDepositsInBanks	259600000	usd
CY2023Q4	us-gaap	Interest Bearing Deposits In Banks InterestBearingDepositsInBanks	199700000	usd
CY2024Q3	us-gaap	Federal Funds Sold FederalFundsSold	100000	usd
CY2023Q4	us-gaap	Federal Funds Sold FederalFundsSold	100000	usd
CY2023Q4	us-gaap	Cash Cash Equivalents And Federal Funds Sold CashCashEquivalentsAndFederalFundsSold	578000000.0	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	5534300000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	5841500000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	2480500000	usd
CY2023Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	2874000000	usd
CY2024Q3	us-gaap	Debt Securities Held To Maturity Amortized Cost After Allowance For Credit Loss DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss	2741300000	usd
CY2023Q4	us-gaap	Debt Securities Held To Maturity Amortized Cost After Allowance For Credit Loss DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss	3207900000	usd
CY2024Q3	us-gaap	Investments Investments	8275600000	usd
CY2023Q4	us-gaap	Investments Investments	9049400000	usd
CY2024Q3	us-gaap	Federal Home Loan Bank Stock And Federal Reserve Bank Stock FederalHomeLoanBankStockAndFederalReserveBankStock	155500000	usd
CY2023Q4	us-gaap	Federal Home Loan Bank Stock And Federal Reserve Bank Stock FederalHomeLoanBankStockAndFederalReserveBankStock	223200000	usd
CY2024Q3	us-gaap	Mortgages Held For Sale Fair Value Disclosure MortgagesHeldForSaleFairValueDisclosure	20900000	usd
CY2023Q4	us-gaap	Mortgages Held For Sale Fair Value Disclosure MortgagesHeldForSaleFairValueDisclosure	47400000	usd
CY2024Q3	us-gaap	Loans And Leases Receivable Net Of Deferred Income LoansAndLeasesReceivableNetOfDeferredIncome	18027100000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Net Of Deferred Income LoansAndLeasesReceivableNetOfDeferredIncome	18279600000	usd
CY2024Q3	us-gaap	Loans And Leases Receivable Allowance LoansAndLeasesReceivableAllowance	225400000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Allowance LoansAndLeasesReceivableAllowance	227700000	usd
CY2024Q3	us-gaap	Loans And Leases Receivable Net Reported Amount LoansAndLeasesReceivableNetReportedAmount	17801700000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Net Reported Amount LoansAndLeasesReceivableNetReportedAmount	18051900000	usd
CY2024Q3	us-gaap	Goodwill Goodwill	1100900000	usd
CY2023Q4	us-gaap	Goodwill Goodwill	1100900000	usd
CY2024Q3	us-gaap	Bank Owned Life Insurance BankOwnedLifeInsurance	511000000.0	usd
CY2023Q4	us-gaap	Bank Owned Life Insurance BankOwnedLifeInsurance	502400000	usd
CY2024Q3	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	432700000	usd
CY2023Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	444300000	usd
CY2024Q3	us-gaap	Indefinite Lived Intangible Assets Excluding Goodwill IndefiniteLivedIntangibleAssetsExcludingGoodwill	70400000	usd
CY2023Q4	us-gaap	Indefinite Lived Intangible Assets Excluding Goodwill IndefiniteLivedIntangibleAssetsExcludingGoodwill	81400000	usd
CY2024Q3	us-gaap	Interest Receivable InterestReceivable	127300000	usd
CY2023Q4	us-gaap	Interest Receivable InterestReceivable	129100000	usd
CY2024Q3	us-gaap	Servicing Asset At Amortized Value ServicingAssetAtAmortizedValue	26300000	usd
CY2023Q4	us-gaap	Servicing Asset At Amortized Value ServicingAssetAtAmortizedValue	28300000	usd
CY2024Q3	us-gaap	Other Real Estate OtherRealEstate	4400000	usd
CY2023Q4	us-gaap	Other Real Estate OtherRealEstate	16500000	usd
CY2024Q3	us-gaap	Deferred Income Tax Assets Net DeferredIncomeTaxAssetsNet	110900000	usd
CY2023Q4	us-gaap	Deferred Income Tax Assets Net DeferredIncomeTaxAssetsNet	150000000.0	usd
CY2024Q3	us-gaap	Other Assets OtherAssets	259300000	usd
CY2023Q4	us-gaap	Other Assets OtherAssets	268400000	usd
CY2024Q3	us-gaap	Assets Assets	29595500000	usd
CY2024Q3	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	5919000000	usd
CY2023Q4	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	6029600000	usd
CY2024Q3	us-gaap	Interest Bearing Deposit Liabilities InterestBearingDepositLiabilities	16945100000	usd
CY2023Q4	us-gaap	Interest Bearing Deposit Liabilities InterestBearingDepositLiabilities	17293500000	usd
CY2024Q3	us-gaap	Deposits Deposits	22864100000	usd
CY2023Q4	us-gaap	Deposits Deposits	23323100000	usd
CY2024Q3	us-gaap	Securities Sold Under Agreements To Repurchase SecuritiesSoldUnderAgreementsToRepurchase	557200000	usd
CY2023Q4	us-gaap	Securities Sold Under Agreements To Repurchase SecuritiesSoldUnderAgreementsToRepurchase	782700000	usd
CY2024Q3	us-gaap	Accounts Payable And Accrued Liabilities Current And Noncurrent AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent	334700000	usd
CY2023Q4	us-gaap	Accounts Payable And Accrued Liabilities Current And Noncurrent AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent	380400000	usd
CY2024Q3	us-gaap	Interest Payable Current And Noncurrent InterestPayableCurrentAndNoncurrent	87700000	usd
CY2024Q3	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2080000000	usd
CY2023Q4	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2603000000	usd
CY2024Q3	us-gaap	Other Long Term Debt OtherLongTermDebt	137300000	usd
CY2024Q3	us-gaap	Allowance For Doubtful Accounts Receivable Current AllowanceForDoubtfulAccountsReceivableCurrent	5600000	usd
CY2023Q4	us-gaap	Allowance For Doubtful Accounts Receivable Current AllowanceForDoubtfulAccountsReceivableCurrent	18400000	usd
CY2024Q3	us-gaap	Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust	163100000	usd
CY2023Q4	us-gaap	Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust	163100000	usd
CY2024Q3	us-gaap	Liabilities Liabilities	26229700000	usd
CY2023Q4	us-gaap	Liabilities Liabilities	27443700000	usd
CY2023Q4	us-gaap	Preferred Stock No Par Value PreferredStockNoParValue	0
CY2024Q3	us-gaap	Preferred Stock No Par Value PreferredStockNoParValue	0
CY2023Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	100000	shares
CY2024Q3	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	100000	shares
CY2023Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2024Q3	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2024Q3	us-gaap	Preferred Stock Including Additional Paid In Capital PreferredStockIncludingAdditionalPaidInCapital	0	usd
CY2023Q4	us-gaap	Preferred Stock Including Additional Paid In Capital PreferredStockIncludingAdditionalPaidInCapital	0	usd
CY2024Q3	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.00001
CY2023Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.00001
CY2024Q3	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	150000000	shares
CY2023Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	103941626	shares
CY2023Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	103941626	shares
CY2023Q4	us-gaap	Common Stocks Including Additional Paid In Capital CommonStocksIncludingAdditionalPaidInCapital	2448900000	usd
CY2023Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	1135100000	usd
CY2023Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-356500000	usd
CY2023Q4	us-gaap	Stockholders Equity StockholdersEquity	3227500000	usd
CY2023Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	30671200000	usd
CY2023Q3	us-gaap	Interest And Fee Income Loans And Leases InterestAndFeeIncomeLoansAndLeases	249900000	usd
	us-gaap	Interest And Fee Income Loans And Leases InterestAndFeeIncomeLoansAndLeases	763800000	usd
	us-gaap	Interest And Fee Income Loans And Leases InterestAndFeeIncomeLoansAndLeases	727100000	usd
CY2023Q3	us-gaap	Interest Income Securities Taxable InterestIncomeSecuritiesTaxable	66000000.0	usd
	us-gaap	Interest Income Securities Tax Exempt InterestIncomeSecuritiesTaxExempt	2500000	usd
CY2023Q3	us-gaap	Other Interest And Dividend Income OtherInterestAndDividendIncome	2900000	usd
	us-gaap	Other Interest And Dividend Income OtherInterestAndDividendIncome	9400000	usd
	us-gaap	Other Interest And Dividend Income OtherInterestAndDividendIncome	9300000	usd
CY2023Q3	us-gaap	Interest Income Domestic Deposits InterestIncomeDomesticDeposits	3000000.0	usd
	us-gaap	Interest Income Domestic Deposits InterestIncomeDomesticDeposits	13900000	usd
	us-gaap	Interest Income Domestic Deposits InterestIncomeDomesticDeposits	12600000	usd
CY2023Q3	us-gaap	Interest And Dividend Income Operating InterestAndDividendIncomeOperating	322600000	usd
	us-gaap	Interest And Dividend Income Operating InterestAndDividendIncomeOperating	976700000	usd
	us-gaap	Interest And Dividend Income Operating InterestAndDividendIncomeOperating	955800000	usd
CY2023Q3	us-gaap	Interest Expense Deposits InterestExpenseDeposits	68800000	usd
	us-gaap	Interest Expense Deposits InterestExpenseDeposits	244200000	usd
	us-gaap	Interest Expense Deposits InterestExpenseDeposits	162700000	usd
CY2023Q3	us-gaap	Interest Expense Securities Sold Under Agreements To Repurchase InterestExpenseSecuritiesSoldUnderAgreementsToRepurchase	1700000	usd
	us-gaap	Interest Expense Securities Sold Under Agreements To Repurchase InterestExpenseSecuritiesSoldUnderAgreementsToRepurchase	5600000	usd
	us-gaap	Interest Expense Securities Sold Under Agreements To Repurchase InterestExpenseSecuritiesSoldUnderAgreementsToRepurchase	4300000	usd
CY2023Q3	us-gaap	Interest Expense Other Short Term Borrowings InterestExpenseOtherShortTermBorrowings	33600000	usd
	us-gaap	Interest Expense Other Short Term Borrowings InterestExpenseOtherShortTermBorrowings	99400000	usd
	us-gaap	Interest Expense Other Short Term Borrowings InterestExpenseOtherShortTermBorrowings	104100000	usd
CY2023Q3	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	1500000	usd
	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	10300000	usd
	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	4400000	usd
CY2023Q3	us-gaap	Interest Expense Junior Subordinated Debentures InterestExpenseJuniorSubordinatedDebentures	3300000	usd
	us-gaap	Interest Expense Junior Subordinated Debentures InterestExpenseJuniorSubordinatedDebentures	9900000	usd
	us-gaap	Interest Expense Junior Subordinated Debentures InterestExpenseJuniorSubordinatedDebentures	9300000	usd
CY2023Q3	us-gaap	Interest Expense InterestExpense	108900000	usd
	us-gaap	Interest Expense InterestExpense	369400000	usd
	us-gaap	Interest Expense InterestExpense	284800000	usd
CY2023Q3	us-gaap	Interest Income Expense Net InterestIncomeExpenseNet	213700000	usd
	us-gaap	Interest Income Expense Net InterestIncomeExpenseNet	607300000	usd
	us-gaap	Interest Income Expense Net InterestIncomeExpenseNet	671000000.0	usd
CY2023Q3	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	-100000	usd
	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	34100000	usd
	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	26800000	usd
CY2023Q3	us-gaap	Interest Income Expense After Provision For Loan Loss InterestIncomeExpenseAfterProvisionForLoanLoss	213800000	usd
	us-gaap	Interest Income Expense After Provision For Loan Loss InterestIncomeExpenseAfterProvisionForLoanLoss	573200000	usd
	us-gaap	Interest Income Expense After Provision For Loan Loss InterestIncomeExpenseAfterProvisionForLoanLoss	644200000	usd
CY2023Q3	fibk	Payment Services Revenues PaymentServicesRevenues	19200000	usd
	fibk	Payment Services Revenues PaymentServicesRevenues	55700000	usd
CY2023Q3	us-gaap	Fees And Commissions Mortgage Banking And Servicing FeesAndCommissionsMortgageBankingAndServicing	2000000.0	usd
	us-gaap	Fees And Commissions Mortgage Banking And Servicing FeesAndCommissionsMortgageBankingAndServicing	5100000	usd
	us-gaap	Fees And Commissions Mortgage Banking And Servicing FeesAndCommissionsMortgageBankingAndServicing	6900000	usd
CY2023Q3	fibk	Wealth Management Revenues WealthManagementRevenues	8700000	usd
	fibk	Wealth Management Revenues WealthManagementRevenues	28200000	usd
	fibk	Wealth Management Revenues WealthManagementRevenues	26500000	usd
	fibk	Service Charges On Deposit Accounts ServiceChargesOnDepositAccounts	19000000.0	usd
	fibk	Service Charges On Deposit Accounts ServiceChargesOnDepositAccounts	17000000.0	usd
CY2023Q3	fibk	Other Service Charges Commissions And Fees OtherServiceChargesCommissionsAndFees	2200000	usd
	fibk	Other Service Charges Commissions And Fees OtherServiceChargesCommissionsAndFees	6500000	usd
	fibk	Other Service Charges Commissions And Fees OtherServiceChargesCommissionsAndFees	7000000.0	usd
CY2023Q3	us-gaap	Debt Securities Available For Sale Gain Loss DebtSecuritiesAvailableForSaleGainLoss	0	usd
	us-gaap	Debt Securities Available For Sale Gain Loss DebtSecuritiesAvailableForSaleGainLoss	0	usd
	us-gaap	Debt Securities Available For Sale Gain Loss DebtSecuritiesAvailableForSaleGainLoss	-23500000	usd
CY2023Q3	us-gaap	Noninterest Income Other Operating Income NoninterestIncomeOtherOperatingIncome	3900000	usd
	us-gaap	Noninterest Income Other Operating Income NoninterestIncomeOtherOperatingIncome	16600000	usd
CY2023Q3	us-gaap	Noninterest Income NoninterestIncome	42000000.0	usd
CY2023Q3	us-gaap	Labor And Related Expense LaborAndRelatedExpense	65400000	usd
	us-gaap	Labor And Related Expense LaborAndRelatedExpense	202400000	usd
	us-gaap	Other Labor Related Expenses OtherLaborRelatedExpenses	61800000	usd
	us-gaap	Information Technology And Data Processing InformationTechnologyAndDataProcessing	42200000	usd
	us-gaap	Information Technology And Data Processing InformationTechnologyAndDataProcessing	44500000	usd
CY2023Q3	us-gaap	Occupancy Net OccupancyNet	11700000	usd
	us-gaap	Occupancy Net OccupancyNet	35800000	usd
	us-gaap	Occupancy Net OccupancyNet	35900000	usd
CY2023Q3	us-gaap	Equipment Expense EquipmentExpense	5300000	usd
	us-gaap	Equipment Expense EquipmentExpense	15400000	usd
	us-gaap	Equipment Expense EquipmentExpense	16800000	usd
CY2023Q3	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	500000	usd
	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	4000000.0	usd
	fibk	Other Real Estate Owned Expense Net Of Income OtherRealEstateOwnedExpenseNetOfIncome	1300000	usd
CY2023Q3	us-gaap	Professional Fees ProfessionalFees	5100000	usd
	us-gaap	Professional Fees ProfessionalFees	16100000	usd
	us-gaap	Professional Fees ProfessionalFees	14900000	usd
CY2023Q3	us-gaap	Federal Deposit Insurance Corporation Premium Expense FederalDepositInsuranceCorporationPremiumExpense	5000000.0	usd
	us-gaap	Federal Deposit Insurance Corporation Premium Expense FederalDepositInsuranceCorporationPremiumExpense	19200000	usd
	us-gaap	Federal Deposit Insurance Corporation Premium Expense FederalDepositInsuranceCorporationPremiumExpense	15400000	usd
CY2023Q3	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	3900000	usd
	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	11000000.0	usd
CY2023Q3	us-gaap	Other Noninterest Expense OtherNoninterestExpense	30000000.0	usd
	us-gaap	Other Noninterest Expense OtherNoninterestExpense	74500000	usd
	us-gaap	Other Noninterest Expense OtherNoninterestExpense	89300000	usd
CY2023Q3	us-gaap	Noninterest Expense NoninterestExpense	161100000	usd
	us-gaap	Noninterest Expense NoninterestExpense	476500000	usd
	us-gaap	Noninterest Expense NoninterestExpense	490800000	usd
CY2023Q3	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	94700000	usd
	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	227800000	usd
	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	255900000	usd
CY2023Q3	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	22000000.0	usd
	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	53900000	usd
	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	59900000	usd
CY2023Q3	us-gaap	Net Income Loss NetIncomeLoss	72700000	usd
	us-gaap	Net Income Loss NetIncomeLoss	173900000	usd
	us-gaap	Net Income Loss NetIncomeLoss	196000000.0	usd
CY2023Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.70
	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.69
	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.89
CY2023Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.70
	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.69
	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.89
	us-gaap	Net Income Loss NetIncomeLoss	173900000	usd
	us-gaap	Net Income Loss NetIncomeLoss	196000000.0	usd
	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	134600000	usd
	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	-51400000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	0	usd
	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	0	usd
	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	-23500000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	0	usd
	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	0	usd
	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	-7200000	usd
CY2023Q3	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Before Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossBeforeTax	300000	usd
	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Before Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossBeforeTax	500000	usd
	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Before Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossBeforeTax	1100000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification And Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax	-13100000	usd
	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification And Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax	-10100000	usd
	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification And Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax	-27200000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Before Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax	-2100000	usd
	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Before Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax	-11200000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-83400000	usd
	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	135200000	usd
	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-59300000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-20700000	usd
	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	33600000	usd
	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-14700000	usd
	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	101600000	usd
	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-44600000	usd
CY2023Q3	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	10000000.0	usd
	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	275500000	usd
	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	151400000	usd
	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	26800000	usd
	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	1700000	usd
	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	40500000	usd
	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	-1700000	usd
	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	-23500000	usd
	us-gaap	Gain Loss On Sale Of Loans And Leases GainLossOnSaleOfLoansAndLeases	2300000	usd
	us-gaap	Gains Losses On Sales Of Other Real Estate GainsLossesOnSalesOfOtherRealEstate	100000	usd
	fibk	Impairmentof Real Estateand Other Assets Pending Disposal ImpairmentofRealEstateandOtherAssetsPendingDisposal	1100000	usd
	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	9900000	usd
	us-gaap	Life Insurance Corporate Or Bank Owned Change In Value LifeInsuranceCorporateOrBankOwnedChangeInValue	8100000	usd
	us-gaap	Share Based Compensation ShareBasedCompensation	8600000	usd
	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	250800000	usd
	us-gaap	Increase Decrease In Accrued Interest Receivable Net IncreaseDecreaseInAccruedInterestReceivableNet	10900000	usd
	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	-38500000	usd
	us-gaap	Increase Decrease In Interest Payable Net IncreaseDecreaseInInterestPayableNet	49200000	usd
	us-gaap	Increase Decrease In Accounts Payable And Accrued Liabilities IncreaseDecreaseInAccountsPayableAndAccruedLiabilities	-19100000	usd
	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	358700000	usd
	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	0	usd
	us-gaap	Proceeds From Sale And Maturity Of Held To Maturity Securities ProceedsFromSaleAndMaturityOfHeldToMaturitySecurities	174000000.0	usd
	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	1276300000	usd
	fibk	Purchases Of Federal Home Loan Bank And Federal Reserve Bank Stock PurchasesOfFederalHomeLoanBankAndFederalReserveBankStock	187300000	usd
	fibk	Proceeds From Federal Home Loan Bank And Federal Reserve Bank Stock ProceedsFromFederalHomeLoanBankAndFederalReserveBankStock	196400000	usd
	us-gaap	Proceeds From Life Insurance Policies ProceedsFromLifeInsurancePolicies	5200000	usd
	us-gaap	Payments For Proceeds From Other Loans And Leases PaymentsForProceedsFromOtherLoansAndLeases	118100000	usd
	us-gaap	Proceeds From Sale Of Wholly Owned Real Estate And Real Estate Acquired In Settlement Of Loans ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans	2600000	usd
	us-gaap	Payments For Proceeds From Productive Assets PaymentsForProceedsFromProductiveAssets	19400000	usd
	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	1329700000	usd
	us-gaap	Increase Decrease In Deposits IncreaseDecreaseInDeposits	-1394100000	usd
	us-gaap	Proceeds From Payments For In Securities Sold Under Agreements To Repurchase ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase	-163400000	usd
	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	-260000000.0	usd
	us-gaap	Repayments Of Other Long Term Debt RepaymentsOfOtherLongTermDebt	100000	usd
	us-gaap	Proceeds From Issuance Of Unsecured Debt ProceedsFromIssuanceOfUnsecuredDebt	0	usd
	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	1800000	usd
	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	146400000	usd
	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-1965800000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	870500000	usd
	us-gaap	Income Taxes Paid IncomeTaxesPaid	39000000.0	usd
	us-gaap	Interest Paid Net InterestPaidNet	235600000	usd
	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	6400000	usd
	us-gaap	Held To Maturity Securities Transferred Security Unrealized Gain Loss HeldToMaturitySecuritiesTransferredSecurityUnrealizedGainLoss	23000000.0	usd
	us-gaap	Transfer Of Portfolio Loans And Leases To Held For Sale1 TransferOfPortfolioLoansAndLeasesToHeldForSale1	3100000	usd
	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	2500000	usd
	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	900000	usd
CY2023Q4	fibk	Available For Sale And Held To Maturity Securities In Unrealized Loss Positions Qualitative Disclosure Number Of Positions AvailableForSaleAndHeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions	704	investments
	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	81-0331430
CY2024Q3	us-gaap	Cash Cash Equivalents And Federal Funds Sold CashCashEquivalentsAndFederalFundsSold	698600000	usd
CY2023Q4	us-gaap	Assets Assets	30671200000	usd
CY2023Q4	us-gaap	Interest Payable Current And Noncurrent InterestPayableCurrentAndNoncurrent	52200000	usd
CY2024Q3	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2023Q4	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2024Q3	dei	Entity Common Stock Shares Outstanding EntityCommonStockSharesOutstanding	104529863	shares
CY2024Q3	us-gaap	Common Stocks Including Additional Paid In Capital CommonStocksIncludingAdditionalPaidInCapital	2457400000	usd
CY2024Q3	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	1163300000	usd
CY2024Q3	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-254900000	usd
CY2024Q3	us-gaap	Stockholders Equity StockholdersEquity	3365800000	usd
CY2024Q3	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	29595500000	usd
CY2024Q3	us-gaap	Interest And Fee Income Loans And Leases InterestAndFeeIncomeLoansAndLeases	258900000	usd
CY2024Q3	us-gaap	Interest Income Securities Taxable InterestIncomeSecuritiesTaxable	60700000	usd
	us-gaap	Interest Income Securities Tax Exempt InterestIncomeSecuritiesTaxExempt	2100000	usd
CY2024Q3	us-gaap	Other Interest And Dividend Income OtherInterestAndDividendIncome	2800000	usd
CY2024Q3	us-gaap	Interest Income Domestic Deposits InterestIncomeDomesticDeposits	4900000	usd
CY2024Q3	us-gaap	Interest And Dividend Income Operating InterestAndDividendIncomeOperating	328000000.0	usd
CY2024Q3	us-gaap	Interest Expense Deposits InterestExpenseDeposits	84200000	usd
CY2024Q3	us-gaap	Interest Expense Securities Sold Under Agreements To Repurchase InterestExpenseSecuritiesSoldUnderAgreementsToRepurchase	1400000	usd
CY2024Q3	us-gaap	Interest Expense Other Short Term Borrowings InterestExpenseOtherShortTermBorrowings	32000000.0	usd
CY2024Q3	us-gaap	Interest Expense Other Long Term Debt InterestExpenseOtherLongTermDebt	1600000	usd
CY2024Q3	us-gaap	Interest Expense Junior Subordinated Debentures InterestExpenseJuniorSubordinatedDebentures	3300000	usd
CY2024Q3	us-gaap	Interest Expense InterestExpense	122500000	usd
CY2024Q3	us-gaap	Interest Income Expense Net InterestIncomeExpenseNet	205500000	usd
CY2024Q3	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	19800000	usd
CY2024Q3	us-gaap	Interest Income Expense After Provision For Loan Loss InterestIncomeExpenseAfterProvisionForLoanLoss	185700000	usd
CY2024Q3	fibk	Payment Services Revenues PaymentServicesRevenues	18700000	usd
	fibk	Payment Services Revenues PaymentServicesRevenues	58000000.0	usd
CY2024Q3	us-gaap	Fees And Commissions Mortgage Banking And Servicing FeesAndCommissionsMortgageBankingAndServicing	1700000	usd
CY2024Q3	fibk	Wealth Management Revenues WealthManagementRevenues	9600000	usd
CY2024Q3	fibk	Service Charges On Deposit Accounts ServiceChargesOnDepositAccounts	6600000	usd
CY2023Q3	fibk	Service Charges On Deposit Accounts ServiceChargesOnDepositAccounts	6000000.0	usd
CY2024Q3	fibk	Other Service Charges Commissions And Fees OtherServiceChargesCommissionsAndFees	2200000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Gain Loss DebtSecuritiesAvailableForSaleGainLoss	0	usd
CY2024Q3	us-gaap	Noninterest Income Other Operating Income NoninterestIncomeOtherOperatingIncome	7600000	usd
CY2024Q3	us-gaap	Noninterest Income NoninterestIncome	46400000	usd
CY2024Q3	us-gaap	Labor And Related Expense LaborAndRelatedExpense	70900000	usd
CY2024Q3	us-gaap	Equipment Expense EquipmentExpense	5200000	usd
CY2024Q3	us-gaap	Professional Fees ProfessionalFees	4200000	usd
CY2024Q3	us-gaap	Federal Deposit Insurance Corporation Premium Expense FederalDepositInsuranceCorporationPremiumExpense	5200000	usd
CY2024Q3	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	3600000	usd
	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	11800000	usd
CY2024Q3	us-gaap	Other Noninterest Expense OtherNoninterestExpense	24200000	usd
CY2024Q3	us-gaap	Noninterest Expense NoninterestExpense	159400000	usd
CY2024Q3	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	72700000	usd
CY2024Q3	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	17200000	usd
CY2024Q3	us-gaap	Net Income Loss NetIncomeLoss	55500000	usd
CY2024Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.54
CY2024Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.54
CY2024Q3	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	158500000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	-72100000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	0	usd
CY2024Q3	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	0	usd
CY2024Q3	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Before Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossBeforeTax	200000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification And Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax	12400000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Before Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax	-3100000	usd
	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Before Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax	-4100000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	173800000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	43200000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	130600000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-62700000	usd
CY2024Q3	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	186100000	usd
CY2024Q2	us-gaap	Stockholders Equity StockholdersEquity	3225300000	usd
CY2024Q3	us-gaap	Net Income Loss NetIncomeLoss	55500000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	130600000	usd
CY2024Q3	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	3972	shares
CY2024Q3	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	100000	usd
CY2024Q3	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	4881	shares
CY2024Q3	us-gaap	Stock Issued During Period Value Share Based Compensation StockIssuedDuringPeriodValueShareBasedCompensation	0	usd
CY2024Q3	us-gaap	Stock Issued During Period Shares Restricted Stock Award Forfeited StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited	31664	shares
CY2024Q3	us-gaap	Stock Issued During Period Value Share Based Compensation Forfeited StockIssuedDuringPeriodValueShareBasedCompensationForfeited	0	usd
CY2024Q3	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	3600000	usd
CY2024Q3	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	0.47
CY2024Q3	us-gaap	Dividends Common Stock Cash DividendsCommonStockCash	49100000	usd
CY2024Q3	us-gaap	Stockholders Equity StockholdersEquity	3365800000	usd
CY2023Q2	us-gaap	Stockholders Equity StockholdersEquity	3121100000	usd
CY2023Q3	us-gaap	Net Income Loss NetIncomeLoss	72700000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-62700000	usd
CY2023Q3	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	494	shares
CY2023Q3	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	0	usd
CY2023Q3	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	12663	shares
CY2023Q3	us-gaap	Stock Issued During Period Value Share Based Compensation StockIssuedDuringPeriodValueShareBasedCompensation	0	usd
CY2023Q3	us-gaap	Stock Issued During Period Shares Restricted Stock Award Forfeited StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited	21481	shares
CY2023Q3	us-gaap	Stock Issued During Period Value Share Based Compensation Forfeited StockIssuedDuringPeriodValueShareBasedCompensationForfeited	0	usd
CY2023Q3	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	3600000	usd
CY2023Q3	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	0.47
CY2023Q3	us-gaap	Dividends Common Stock Cash DividendsCommonStockCash	49200000	usd
CY2023Q3	us-gaap	Stockholders Equity StockholdersEquity	3085500000	usd
CY2023Q4	us-gaap	Stockholders Equity StockholdersEquity	3227500000	usd
	us-gaap	Net Income Loss NetIncomeLoss	173900000	usd
	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	101600000	usd
	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	49420	shares
	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	1200000	usd
	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	742979	shares
	us-gaap	Stock Issued During Period Value Share Based Compensation StockIssuedDuringPeriodValueShareBasedCompensation	0	usd
	us-gaap	Stock Issued During Period Shares Restricted Stock Award Forfeited StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited	105322	shares
	us-gaap	Stock Issued During Period Value Share Based Compensation Forfeited StockIssuedDuringPeriodValueShareBasedCompensationForfeited	0	usd
	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	9700000	usd
	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.41
	us-gaap	Dividends Common Stock Cash DividendsCommonStockCash	146900000	usd
CY2024Q3	us-gaap	Stockholders Equity StockholdersEquity	3365800000	usd
CY2022Q4	us-gaap	Stockholders Equity StockholdersEquity	3073800000	usd
	us-gaap	Net Income Loss NetIncomeLoss	196000000.0	usd
	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-44600000	usd
	us-gaap	Stock Repurchased And Retired During Period Shares StockRepurchasedAndRetiredDuringPeriodShares	55922	shares
	us-gaap	Stock Repurchased And Retired During Period Value StockRepurchasedAndRetiredDuringPeriodValue	1900000	usd
	us-gaap	Stock Issued During Period Shares Restricted Stock Award Gross StockIssuedDuringPeriodSharesRestrictedStockAwardGross	683574	shares
	us-gaap	Stock Issued During Period Value Share Based Compensation StockIssuedDuringPeriodValueShareBasedCompensation	0	usd
	us-gaap	Stock Issued During Period Shares Restricted Stock Award Forfeited StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited	58304	shares
	us-gaap	Stock Issued During Period Value Share Based Compensation Forfeited StockIssuedDuringPeriodValueShareBasedCompensationForfeited	0	usd
	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	8600000	usd
	us-gaap	Common Stock Dividends Per Share Declared CommonStockDividendsPerShareDeclared	1.41
	us-gaap	Dividends Common Stock Cash DividendsCommonStockCash	146400000	usd
CY2023Q3	us-gaap	Stockholders Equity StockholdersEquity	3085500000	usd
	us-gaap	Net Income Loss NetIncomeLoss	173900000	usd
	us-gaap	Net Income Loss NetIncomeLoss	196000000.0	usd
	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	34100000	usd
	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	0	usd
	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	41400000	usd
	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	-900000	usd
	us-gaap	Debt And Equity Securities Gain Loss DebtAndEquitySecuritiesGainLoss	0	usd
	us-gaap	Gain Loss On Sale Of Loans And Leases GainLossOnSaleOfLoansAndLeases	1400000	usd
	us-gaap	Gains Losses On Sales Of Other Real Estate GainsLossesOnSalesOfOtherRealEstate	400000	usd
	fibk	Impairmentof Real Estateand Other Assets Pending Disposal ImpairmentofRealEstateandOtherAssetsPendingDisposal	4200000	usd
	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	6600000	usd
	us-gaap	Life Insurance Corporate Or Bank Owned Change In Value LifeInsuranceCorporateOrBankOwnedChangeInValue	10700000	usd
	us-gaap	Share Based Compensation ShareBasedCompensation	9700000	usd
	us-gaap	Payments For Origination Of Mortgage Loans Held For Sale PaymentsForOriginationOfMortgageLoansHeldForSale	131400000	usd
	us-gaap	Proceeds From Sale Of Mortgage Loans Held For Sale ProceedsFromSaleOfMortgageLoansHeldForSale	130700000	usd
	us-gaap	Proceeds From Sale Of Mortgage Loans Held For Sale ProceedsFromSaleOfMortgageLoansHeldForSale	255900000	usd
	us-gaap	Increase Decrease In Accrued Interest Receivable Net IncreaseDecreaseInAccruedInterestReceivableNet	-1800000	usd
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	us-gaap	Increase Decrease In Interest Payable Net IncreaseDecreaseInInterestPayableNet	35500000	usd
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	us-gaap	Proceeds From Sale And Maturity Of Held To Maturity Securities ProceedsFromSaleAndMaturityOfHeldToMaturitySecurities	470000000.0	usd
	us-gaap	Proceeds From Sale And Maturity Of Available For Sale Securities ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	539400000	usd
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	fibk	Proceeds From Federal Home Loan Bank And Federal Reserve Bank Stock ProceedsFromFederalHomeLoanBankAndFederalReserveBankStock	160100000	usd
	us-gaap	Proceeds From Life Insurance Policies ProceedsFromLifeInsurancePolicies	2100000	usd
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	us-gaap	Proceeds From Sale Of Wholly Owned Real Estate And Real Estate Acquired In Settlement Of Loans ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans	12900000	usd
	us-gaap	Payments For Proceeds From Productive Assets PaymentsForProceedsFromProductiveAssets	16400000	usd
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	us-gaap	Proceeds From Payments For In Securities Sold Under Agreements To Repurchase ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase	-225500000	usd
	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	-773000000.0	usd
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	us-gaap	Payments For Repurchase Of Common Stock PaymentsForRepurchaseOfCommonStock	1200000	usd
	us-gaap	Payments Of Dividends Common Stock PaymentsOfDividendsCommonStock	146900000	usd
CY2023Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	578000000.0	usd
	us-gaap	Income Taxes Paid IncomeTaxesPaid	12100000	usd
	us-gaap	Interest Paid Net InterestPaidNet	333900000	usd
	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	1100000	usd
	us-gaap	Held To Maturity Securities Transferred Security Unrealized Gain Loss HeldToMaturitySecuritiesTransferredSecurityUnrealizedGainLoss	0	usd
	us-gaap	Transfer Of Portfolio Loans And Leases To Held For Sale1 TransferOfPortfolioLoansAndLeasesToHeldForSale1	0	usd
	us-gaap	Transfer To Other Real Estate TransferToOtherRealEstate	4600000	usd
	fibk	Capitalization Of Internally Originated Mortgage Servicing Rights Noncash Supplemental Disclosures CapitalizationOfInternallyOriginatedMortgageServicingRightsNoncashSupplementalDisclosures	500000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Amortized Cost Excluding Accrued Interest Before Allowance For Credit Loss DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss	5866000000	usd
CY2024Q3	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	6600000	usd
CY2024Q3	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	338300000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	5534300000	usd
CY2024Q3	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	2742000000	usd
CY2024Q3	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	700000	usd
CY2024Q3	us-gaap	Debt Securities Held To Maturity Amortized Cost After Allowance For Credit Loss DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss	2741300000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Gain HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain	1600000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Loss HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss	262400000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	2480500000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Excluding Accrued Interest Before Allowance For Credit Loss DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss	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
CY2023Q4	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	3208700000	usd
CY2023Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	800000	usd
CY2023Q4	us-gaap	Debt Securities Held To Maturity Amortized Cost After Allowance For Credit Loss DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss	3207900000	usd
CY2023Q4	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Gain HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain	400000	usd
CY2023Q4	us-gaap	Held To Maturity Securities Accumulated Unrecognized Holding Loss HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss	334300000	usd
CY2023Q4	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	2874000000	usd
CY2023	fibk	Debt Securities Held To Maturity Transfer Unamortized Gain DebtSecuritiesHeldToMaturityTransferUnamortizedGain	10700000	usd
CY2023	fibk	Debt Securities Held To Maturity Transfer Unamortized Loss DebtSecuritiesHeldToMaturityTransferUnamortizedLoss	18100000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months	3300000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months Accumulated Loss DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	0	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger	4295700000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer Accumulated Loss DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	338300000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Unrealized Loss Position DebtSecuritiesAvailableForSaleUnrealizedLossPosition	4299000000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Accumulated Loss DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss	338300000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months	900000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months Accumulated Loss DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	0	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger	5505500000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer Accumulated Loss DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	469500000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position DebtSecuritiesAvailableForSaleUnrealizedLossPosition	5506400000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Accumulated Loss DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss	469500000	usd
CY2024Q3	fibk	Available For Sale And Held To Maturity Securities In Unrealized Loss Positions Qualitative Disclosure Number Of Positions AvailableForSaleAndHeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions	648	investments
CY2024Q3	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	2742000000	usd
CY2023Q4	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	3208700000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	0	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	0	usd
CY2023Q3	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	0	usd
CY2023Q3	us-gaap	Debt Securities Available For Sale Realized Gain DebtSecuritiesAvailableForSaleRealizedGain	0	usd
CY2023Q3	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	0	usd
CY2023Q3	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	0	usd
	us-gaap	Debt Securities Available For Sale Realized Loss DebtSecuritiesAvailableForSaleRealizedLoss	-23500000	usd
CY2023Q3	us-gaap	Debt Securities Current DebtSecuritiesCurrent	853000000	usd
CY2024Q3	us-gaap	Available For Sale Securities Debt Maturities Next Rolling Twelve Months Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis	88700000	usd
CY2024Q3	us-gaap	Available For Sale Securities Debt Maturities Next Rolling Twelve Months Fair Value AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue	87500000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Debt Maturities Within One Year Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount	101100000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Debt Maturities Next Rolling Twelve Months Fair Value HeldToMaturitySecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue	100600000	usd
CY2024Q3	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Two Through Five Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis	1326900000	usd
CY2024Q3	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Two Through Five Fair Value AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue	1265900000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Debt Maturities After One Through Five Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount	416000000.0	usd
CY2024Q3	us-gaap	Held To Maturity Securities Debt Maturities Rolling Year Two Through Five Fair Value HeldToMaturitySecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue	396800000	usd
CY2024Q3	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Six Through Ten Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis	1166400000	usd
CY2024Q3	us-gaap	Available For Sale Securities Debt Maturities Rolling Year Six Through Ten Fair Value AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue	1095100000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Debt Maturities After Five Through Ten Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount	485500000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Debt Maturities Rolling Year Six Through Ten Fair Value HeldToMaturitySecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue	437500000	usd
CY2024Q3	us-gaap	Available For Sale Securities Debt Maturities Rolling After Year Ten Amortized Cost Basis AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenAmortizedCostBasis	3284000000	usd
CY2024Q3	us-gaap	Available For Sale Securities Debt Maturities Rolling After Year Ten Fair Value AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenFairValue	3085800000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Debt Maturities After Ten Years Net Carrying Amount HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount	1739400000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Debt Maturities Rolling After Ten Years Fair Value HeldToMaturitySecuritiesDebtMaturitiesRollingAfterTenYearsFairValue	1545600000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Amortized Cost Excluding Accrued Interest Before Allowance For Credit Loss DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss	5866000000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	5534300000	usd
CY2024Q3	us-gaap	Held To Maturity Securities HeldToMaturitySecurities	2742000000	usd
CY2024Q3	us-gaap	Held To Maturity Securities Fair Value HeldToMaturitySecuritiesFairValue	2480500000	usd
CY2024Q3	fibk	Investment Securities Primarily Classified As Available For Sale Callable Within One Year Amortized Cost Basis InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleCallableWithinOneYearAmortizedCostBasis	1514600000	usd
CY2024Q3	fibk	Investment Securities Primarily Classified As Available For Sale Callable Within One Year Fair Value InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleCallableWithinOneYearFairValue	1478000000	usd
CY2024Q3	fibk	Investment Securities Callable Structured Amortized Cost Basis InvestmentSecuritiesCallableStructuredAmortizedCostBasis	0	usd
CY2024Q3	us-gaap	Securities Sold Under Agreements To Repurchase Fair Value Of Collateral SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral	3159800000	usd
CY2023Q4	us-gaap	Securities Sold Under Agreements To Repurchase Fair Value Of Collateral SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral	3858600000	usd
CY2024Q3	us-gaap	Securities Loaned Or Sold Under Agreements To Repurchase Fair Value Disclosure SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure	2888000000	usd
CY2023Q4	us-gaap	Securities Loaned Or Sold Under Agreements To Repurchase Fair Value Disclosure SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure	3462200000	usd
CY2024Q3	us-gaap	Federal Home Loan Bank Stock And Federal Reserve Bank Stock FederalHomeLoanBankStockAndFederalReserveBankStock	155500000	usd
CY2023Q4	us-gaap	Federal Home Loan Bank Stock And Federal Reserve Bank Stock FederalHomeLoanBankStockAndFederalReserveBankStock	223200000	usd
CY2024Q3	us-gaap	Loans Receivable Held For Sale Net Not Part Of Disposal Group LoansReceivableHeldForSaleNetNotPartOfDisposalGroup	20900000	usd
CY2023Q4	us-gaap	Loans Receivable Held For Sale Net Not Part Of Disposal Group LoansReceivableHeldForSaleNetNotPartOfDisposalGroup	47400000	usd
CY2024Q3	us-gaap	Loans And Leases Receivable Deferred Income LoansAndLeasesReceivableDeferredIncome	11800000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Deferred Income LoansAndLeasesReceivableDeferredIncome	12500000	usd
CY2024Q3	us-gaap	Loans And Leases Receivable Net Of Deferred Income LoansAndLeasesReceivableNetOfDeferredIncome	18027100000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Net Of Deferred Income LoansAndLeasesReceivableNetOfDeferredIncome	18279600000	usd
CY2024Q3	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	225400000	usd
CY2023Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	227700000	usd
CY2024Q3	us-gaap	Loans And Leases Receivable Net Reported Amount LoansAndLeasesReceivableNetReportedAmount	17801700000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Net Reported Amount LoansAndLeasesReceivableNetReportedAmount	18051900000	usd
CY2024Q2	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	232800000	usd
CY2024Q3	us-gaap	Off Balance Sheet Credit Loss Liability Credit Loss Expense Reversal OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal	20000000.0	usd
CY2024Q3	us-gaap	Allowance For Loan And Lease Losses Write Offs AllowanceForLoanAndLeaseLossesWriteOffs	29100000	usd
CY2024Q3	us-gaap	Financing Receivable Allowance For Credit Losses Recovery FinancingReceivableAllowanceForCreditLossesRecovery	1700000	usd
CY2024Q3	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	225400000	usd
CY2023Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	227700000	usd
	us-gaap	Off Balance Sheet Credit Loss Liability Credit Loss Expense Reversal OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal	47000000.0	usd
	us-gaap	Allowance For Loan And Lease Losses Write Offs AllowanceForLoanAndLeaseLossesWriteOffs	56400000	usd
	us-gaap	Financing Receivable Allowance For Credit Losses Recovery FinancingReceivableAllowanceForCreditLossesRecovery	7100000	usd
CY2024Q3	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	225400000	usd
CY2023Q2	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	224600000	usd
CY2023Q3	us-gaap	Off Balance Sheet Credit Loss Liability Credit Loss Expense Reversal OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal	3200000	usd
CY2023Q3	us-gaap	Allowance For Loan And Lease Losses Write Offs AllowanceForLoanAndLeaseLossesWriteOffs	6200000	usd
CY2023Q3	us-gaap	Financing Receivable Allowance For Credit Losses Recovery FinancingReceivableAllowanceForCreditLossesRecovery	5100000	usd
CY2023Q3	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	226700000	usd
CY2022Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	220100000	usd
	us-gaap	Off Balance Sheet Credit Loss Liability Credit Loss Expense Reversal OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal	25300000	usd
	us-gaap	Allowance For Loan And Lease Losses Write Offs AllowanceForLoanAndLeaseLossesWriteOffs	29200000	usd
	us-gaap	Financing Receivable Allowance For Credit Losses Recovery FinancingReceivableAllowanceForCreditLossesRecovery	10500000	usd
CY2023Q3	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	226700000	usd
	us-gaap	Impaired Financing Receivable Interest Income Accrual Method ImpairedFinancingReceivableInterestIncomeAccrualMethod	0	usd
	us-gaap	Impaired Financing Receivable Interest Income Accrual Method ImpairedFinancingReceivableInterestIncomeAccrualMethod	0	usd
CY2024Q3	us-gaap	Impaired Financing Receivable Interest Income Accrual Method ImpairedFinancingReceivableInterestIncomeAccrualMethod	0	usd
CY2023Q3	us-gaap	Impaired Financing Receivable Interest Income Accrual Method ImpairedFinancingReceivableInterestIncomeAccrualMethod	0	usd
CY2024Q3	us-gaap	Financing Receivable Accrued Interest Writeoff FinancingReceivableAccruedInterestWriteoff	-400000	usd
	us-gaap	Financing Receivable Accrued Interest Writeoff FinancingReceivableAccruedInterestWriteoff	-2400000	usd
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	us-gaap	Financing Receivable Accrued Interest Writeoff FinancingReceivableAccruedInterestWriteoff	-1900000	usd
CY2024Q3	us-gaap	Financing Receivable Excluding Accrued Interest Modified Period FinancingReceivableExcludingAccruedInterestModifiedPeriod	31700000	usd
CY2024Q3	us-gaap	Financing Receivable Modification To Total Financing Receivables Excluding Accrued Interest Percent FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent	0.0018
	us-gaap	Financing Receivable Excluding Accrued Interest Modified Period FinancingReceivableExcludingAccruedInterestModifiedPeriod	89400000	usd
	us-gaap	Financing Receivable Modification To Total Financing Receivables Excluding Accrued Interest Percent FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent	0.0050
CY2023Q3	us-gaap	Financing Receivable Excluding Accrued Interest Modified Period FinancingReceivableExcludingAccruedInterestModifiedPeriod	33200000	usd
CY2023Q3	us-gaap	Financing Receivable Modification To Total Financing Receivables Excluding Accrued Interest Percent FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent	0.0018
	us-gaap	Financing Receivable Excluding Accrued Interest Modified Period FinancingReceivableExcludingAccruedInterestModifiedPeriod	91200000	usd
	us-gaap	Financing Receivable Modification To Total Financing Receivables Excluding Accrued Interest Percent FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent	0.0050
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	us-gaap	Financing Receivable Accrued Interest Modified FinancingReceivableAccruedInterestModified	1700000	usd
CY2024Q3	us-gaap	Financing Receivable Loan In Process FinancingReceivableLoanInProcess	500000	usd
CY2024	us-gaap	Financing Receivable Excluding Accrued Interest Modified Subsequent Default FinancingReceivableExcludingAccruedInterestModifiedSubsequentDefault	1300000	usd
	us-gaap	Loans And Leases Receivable Commitments To Purchase Or Sell LoansAndLeasesReceivableCommitmentsToPurchaseOrSell	0	usd
	us-gaap	Loans And Leases Receivable Commitments To Purchase Or Sell LoansAndLeasesReceivableCommitmentsToPurchaseOrSell	0	usd
CY2023Q3	us-gaap	Loans And Leases Receivable Commitments To Purchase Or Sell LoansAndLeasesReceivableCommitmentsToPurchaseOrSell	0	usd
CY2024Q3	us-gaap	Loans And Leases Receivable Commitments To Purchase Or Sell LoansAndLeasesReceivableCommitmentsToPurchaseOrSell	0	usd
	us-gaap	Proceeds From Sale Of Loans Held For Investment ProceedsFromSaleOfLoansHeldForInvestment	0	usd
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	us-gaap	Proceeds From Sale Of Loans Held For Investment ProceedsFromSaleOfLoansHeldForInvestment	0	usd
CY2024Q2	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	6700000	usd
CY2023Q2	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	14400000	usd
CY2023Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	16500000	usd
CY2022Q4	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	12700000	usd
CY2024Q3	fibk	Real Estate Acquired Through Foreclosure Additions RealEstateAcquiredThroughForeclosureAdditions	900000	usd
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CY2024Q3	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	0	usd
CY2023Q3	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	300000	usd
	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	4200000	usd
	fibk	Real Estate Acquired Through Foreclosure Valuation Adjustments RealEstateAcquiredThroughForeclosureValuationAdjustments	1100000	usd
CY2024Q3	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	3200000	usd
CY2023Q3	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	2500000	usd
	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	12500000	usd
	fibk	Real Estate Acquired Through Foreclosure Disposals RealEstateAcquiredThroughForeclosureDisposals	2500000	usd
CY2024Q3	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	4400000	usd
CY2023Q3	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	11600000	usd
CY2024Q3	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	4400000	usd
CY2023Q3	us-gaap	Real Estate Acquired Through Foreclosure RealEstateAcquiredThroughForeclosure	11600000	usd
CY2024Q3	us-gaap	Mortgage Loans In Process Of Foreclosure Amount MortgageLoansInProcessOfForeclosureAmount	1700000	usd
CY2023Q4	us-gaap	Mortgage Loans In Process Of Foreclosure Amount MortgageLoansInProcessOfForeclosureAmount	500000	usd
CY2024Q3	us-gaap	Interest Rate Cash Flow Hedge Gain Loss To Be Reclassified During Next12 Months Net InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet	1700000	usd
CY2024Q3	us-gaap	Interest Income Hedge Basis Adjustment Portfolio Layer Method InterestIncomeHedgeBasisAdjustmentPortfolioLayerMethod	400000	usd
CY2024Q3	us-gaap	Derivative Instruments And Hedges DerivativeInstrumentsAndHedges	194300000	usd
CY2024Q3	us-gaap	Hedged Asset Fair Value Hedge Cumulative Increase Decrease HedgedAssetFairValueHedgeCumulativeIncreaseDecrease	5700000	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
CY2024Q3	us-gaap	Derivative Asset Notional Amount DerivativeAssetNotionalAmount	1773800000	usd
CY2024Q3	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	28600000	usd
CY2023Q4	us-gaap	Derivative Asset Notional Amount DerivativeAssetNotionalAmount	2144700000	usd
CY2023Q4	us-gaap	Interest Rate Derivative Assets At Fair Value InterestRateDerivativeAssetsAtFairValue	37400000	usd
CY2024Q3	us-gaap	Derivative Liability Notional Amount DerivativeLiabilityNotionalAmount	1868900000	usd
CY2024Q3	us-gaap	Interest Rate Derivative Liabilities At Fair Value InterestRateDerivativeLiabilitiesAtFairValue	94200000	usd
CY2023Q4	us-gaap	Derivative Liability Notional Amount DerivativeLiabilityNotionalAmount	2295700000	usd
CY2023Q4	us-gaap	Interest Rate Derivative Liabilities At Fair Value InterestRateDerivativeLiabilitiesAtFairValue	127300000	usd
CY2024Q3	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	28600000	usd
CY2024Q3	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	usd
CY2024Q3	us-gaap	Derivative Assets DerivativeAssets	28600000	usd
CY2024Q3	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	usd
CY2024Q3	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	25800000	usd
CY2024Q3	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	2800000	usd
CY2024Q3	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	28600000	usd
CY2024Q3	us-gaap	Derivative Asset Fair Value Gross Liability DerivativeAssetFairValueGrossLiability	0	usd
CY2024Q3	us-gaap	Derivative Assets DerivativeAssets	28600000	usd
CY2024Q3	us-gaap	Derivative Collateral Obligation To Return Securities DerivativeCollateralObligationToReturnSecurities	0	usd
CY2024Q3	us-gaap	Derivative Collateral Obligation To Return Cash DerivativeCollateralObligationToReturnCash	25800000	usd
CY2024Q3	us-gaap	Derivative Fair Value Of Derivative Asset Amount Offset Against Collateral DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral	2800000	usd
CY2024Q3	us-gaap	Derivative Fair Value Of Derivative Liability DerivativeFairValueOfDerivativeLiability	94200000	usd
CY2024Q3	us-gaap	Derivative Liability Fair Value Gross Asset DerivativeLiabilityFairValueGrossAsset	0	usd
CY2024Q3	us-gaap	Derivative Liabilities DerivativeLiabilities	94200000	usd
CY2024Q3	us-gaap	Derivative Collateral Right To Reclaim Securities DerivativeCollateralRightToReclaimSecurities	0	usd
CY2024Q3	us-gaap	Derivative Collateral Right To Reclaim Cash DerivativeCollateralRightToReclaimCash	0	usd
CY2024Q3	us-gaap	Derivative Fair Value Of Derivative Liability Amount Offset Against Collateral DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral	94200000	usd
CY2024Q3	us-gaap	Securities Sold Under Agreements To Repurchase Gross SecuritiesSoldUnderAgreementsToRepurchaseGross	557200000	usd
CY2024Q3	us-gaap	Securities Sold Under Agreements To Repurchase Asset SecuritiesSoldUnderAgreementsToRepurchaseAsset	0	usd
CY2024Q3	us-gaap	Securities Sold Under Agreements To Repurchase SecuritiesSoldUnderAgreementsToRepurchase	557200000	usd
CY2024Q3	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Securities SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimSecurities	0	usd
CY2024Q3	us-gaap	Securities Sold Under Agreements To Repurchase Collateral Right To Reclaim Cash SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimCash	0	usd
CY2024Q3	us-gaap	Securities Sold Under Agreements To Repurchase Amount Offset Against Collateral SecuritiesSoldUnderAgreementsToRepurchaseAmountOffsetAgainstCollateral	557200000	usd
CY2024Q3	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Gross DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedGross	651400000	usd
CY2024Q3	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Asset DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset	0	usd
CY2024Q3	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoaned	651400000	usd
CY2024Q3	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Securities DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimSecurities	0	usd
CY2024Q3	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Collateral Right To Reclaim Cash DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimCash	0	usd
CY2024Q3	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Amount Offset Against Collateral DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAmountOffsetAgainstCollateral	651400000	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	Derivative Assets DerivativeAssets	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	Derivative Assets DerivativeAssets	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	Derivative Liabilities DerivativeLiabilities	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	0	usd
CY2023Q4	us-gaap	Securities Sold Under Agreements To Repurchase Amount Offset Against Collateral SecuritiesSoldUnderAgreementsToRepurchaseAmountOffsetAgainstCollateral	782700000	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	0	usd
CY2023Q4	us-gaap	Derivative Liability Securities Sold Under Agreements To Resell Securities Loaned Amount Offset Against Collateral DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAmountOffsetAgainstCollateral	910000000.0	usd
CY2024Q3	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	137300000	usd
CY2023Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	120800000	usd
CY2024Q3	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2080000000	usd
CY2024Q3	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2080000000	usd
CY2023Q4	us-gaap	Advances From Federal Home Loan Banks AdvancesFromFederalHomeLoanBanks	2603000000	usd
CY2023Q4	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2603000000	usd
CY2024Q3	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2080000000	usd
CY2024Q3	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2080000000	usd
CY2023Q4	us-gaap	Advances From Federal Home Loan Banks AdvancesFromFederalHomeLoanBanks	2603000000	usd
CY2023Q4	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2603000000	usd
CY2024Q3	fibk	Capital Stock Authorized CapitalStockAuthorized	150100000	shares
CY2024Q3	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	150000000	shares
CY2024Q3	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	100000	shares
CY2024Q3	dei	Entity Common Stock Shares Outstanding EntityCommonStockSharesOutstanding	104529863	shares
CY2023Q4	us-gaap	Preferred Stock No Par Value PreferredStockNoParValue	0
CY2024Q3	us-gaap	Preferred Stock No Par Value PreferredStockNoParValue	0
CY2024Q3	us-gaap	Net Income Loss NetIncomeLoss	55500000	usd
CY2023Q3	us-gaap	Net Income Loss NetIncomeLoss	72700000	usd
	us-gaap	Net Income Loss NetIncomeLoss	173900000	usd
	us-gaap	Net Income Loss NetIncomeLoss	196000000.0	usd
CY2024Q3	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	102970851	shares
CY2023Q3	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	103822311	shares
	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	102917610	shares
	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	103793851	shares
CY2024Q3	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	262767	shares
CY2023Q3	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	4116	shares
	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	207449	shares
	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	30425	shares
CY2024Q3	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	103233618	shares
CY2023Q3	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	103826427	shares
	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	103125059	shares
	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	103824276	shares
CY2024Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.54
CY2023Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.70
	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.69
	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.89
CY2024Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.54
CY2023Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.70
	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.69
	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	1.89
CY2024Q3	us-gaap	Capital Capital	2976200000	usd
CY2024Q3	us-gaap	Capital To Risk Weighted Assets CapitalToRiskWeightedAssets	0.1411
CY2024Q3	us-gaap	Capital Required For Capital Adequacy CapitalRequiredForCapitalAdequacy	1687200000	usd
CY2024Q3	us-gaap	Capital Required For Capital Adequacy To Risk Weighted Assets CapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0800
CY2024Q3	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	2214400000	usd
CY2024Q3	fibk	Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets CapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.1050
CY2024Q3	us-gaap	Capital Required To Be Well Capitalized CapitalRequiredToBeWellCapitalized	2109000000	usd
CY2024Q3	us-gaap	Capital Required To Be Well Capitalized To Risk Weighted Assets CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.1000
CY2024Q3	us-gaap	Tier One Risk Based Capital TierOneRiskBasedCapital	2495800000	usd
CY2024Q3	us-gaap	Tier One Risk Based Capital To Risk Weighted Assets TierOneRiskBasedCapitalToRiskWeightedAssets	0.1183
CY2024Q3	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy TierOneRiskBasedCapitalRequiredForCapitalAdequacy	1265400000	usd
CY2024Q3	us-gaap	Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets	0.0600
CY2024Q3	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	1792600000	usd
CY2024Q3	fibk	Tier One Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0850
CY2024Q3	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized TierOneRiskBasedCapitalRequiredToBeWellCapitalized	1687200000	usd
CY2024Q3	us-gaap	Tier One Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	0.0800
CY2024Q3	us-gaap	Common Equity Tier One Capital CommonEquityTierOneCapital	2495800000	usd
CY2024Q3	us-gaap	Common Equity Tier One Capital Ratio CommonEquityTierOneCapitalRatio	0.1183
CY2024Q3	us-gaap	Common Equity Tier One Capital Required For Capital Adequacy CommonEquityTierOneCapitalRequiredForCapitalAdequacy	949000000.0	usd
CY2024Q3	us-gaap	Banking Regulation Common Equity Tier One Risk Based Capital Ratio Capital Adequacy Minimum BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioCapitalAdequacyMinimum	0.0450
CY2024Q3	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedIn	1476300000	usd
CY2024Q3	fibk	Tier One Common Equity Capital Required For Capital Adequacy Under Basel Iii Fully Phased In To Risk Weighted Assets TierOneCommonEquityCapitalRequiredForCapitalAdequacyUnderBaselIIIFullyPhasedInToRiskWeightedAssets	0.0700
CY2024Q3	us-gaap	Common Equity Tier One Capital Required To Be Well Capitalized CommonEquityTierOneCapitalRequiredToBeWellCapitalized	1370800000	usd
CY2024Q3	us-gaap	Banking Regulation Common Equity Tier One Risk Based Capital Ratio Well Capitalized Minimum BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioWellCapitalizedMinimum	0.0650
CY2024Q3	us-gaap	Tier One Leverage Capital TierOneLeverageCapital	2495800000	usd
CY2024Q3	us-gaap	Tier One Leverage Capital To Average Assets TierOneLeverageCapitalToAverageAssets	0.0857
CY2024Q3	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	1164900000	usd
CY2024Q3	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2024Q3	us-gaap	Tier One Leverage Capital Required For Capital Adequacy TierOneLeverageCapitalRequiredForCapitalAdequacy	1164900000	usd
CY2024Q3	us-gaap	Tier One Leverage Capital Required For Capital Adequacy To Average Assets TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets	0.0400
CY2024Q3	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized TierOneLeverageCapitalRequiredToBeWellCapitalized	1456200000	usd
CY2024Q3	us-gaap	Tier One Leverage Capital Required To Be Well Capitalized To Average Assets TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets	0.0500
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	us-gaap	Common Equity Tier One Capital CommonEquityTierOneCapital	2454400000	usd
CY2023Q4	us-gaap	Common Equity Tier One Capital Ratio CommonEquityTierOneCapitalRatio	0.1108
CY2023Q4	us-gaap	Common Equity Tier One Capital Required For Capital Adequacy CommonEquityTierOneCapitalRequiredForCapitalAdequacy	996500000	usd
CY2023Q4	us-gaap	Banking Regulation Common Equity Tier One Risk Based Capital Ratio Capital Adequacy Minimum BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioCapitalAdequacyMinimum	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	us-gaap	Common Equity Tier One Capital Required To Be Well Capitalized CommonEquityTierOneCapitalRequiredToBeWellCapitalized	1439400000	usd
CY2023Q4	us-gaap	Banking Regulation Common Equity Tier One Risk Based Capital Ratio Well Capitalized Minimum BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioWellCapitalizedMinimum	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
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CY2020Q1	us-gaap	Income Tax Effects Allocated Directly To Equity Cumulative Effect Of Change In Accounting Principle IncomeTaxEffectsAllocatedDirectlyToEquityCumulativeEffectOfChangeInAccountingPrinciple	24100000	usd
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CY2022Q4	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	16200000	usd
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CY2023Q3	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	18800000	usd
CY2024Q3	fibk	Credit Extension Commitments CreditExtensionCommitments	3290700000	usd
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CY2024Q3	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	39400000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	-17900000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	119100000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	-54200000	usd
CY2024Q3	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Before Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossBeforeTax	200000	usd
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CY2024Q3	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossTax	0	usd
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CY2024Q3	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss After Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossAfterTax	200000	usd
CY2023Q3	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss After Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossAfterTax	200000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification And Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax	12400000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification And Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax	-13100000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationTax	3000000.0	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationTax	-3200000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification After Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax	9400000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification After Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax	-9900000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Before Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax	-3100000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Before Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax	-2100000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationTax	-800000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationTax	-500000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification After Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax	-2300000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification After Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax	-1600000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	173800000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-83400000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	43200000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-20700000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	130600000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-62700000	usd
	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	134600000	usd
	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax	-51400000	usd
	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	33500000	usd
	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax	-12700000	usd
	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	101100000	usd
	us-gaap	Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	-38700000	usd
	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	0	usd
	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Before Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax	-23500000	usd
	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax	0	usd
	us-gaap	Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Sale Of Securities Tax OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax	-5800000	usd
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	us-gaap	Reclassification From Accumulated Other Comprehensive Income Current Period Net Of Tax ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax	-17700000	usd
	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	0	usd
	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Before Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesBeforeTax	-7200000	usd
	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesTax	0	usd
	us-gaap	Other Comprehensive Income Reclassification Adjustment For Held To Maturity Transferred To Available For Sale Securities Tax OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesTax	-1800000	usd
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	us-gaap	Other Comprehensive Income Loss Transfers From Held To Maturity To Available For Sale Securities Net Of Tax OtherComprehensiveIncomeLossTransfersFromHeldToMaturityToAvailableForSaleSecuritiesNetOfTax	-5400000	usd
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	us-gaap	Oci Debt Securities Available For Sale Transfer To Held To Maturity Adjustment From Aoci For Amortization Of Gain Loss After Tax OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossAfterTax	800000	usd
	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Before Reclassification And Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax	-10100000	usd
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	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification After Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax	-3100000	usd
	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	135200000	usd
	us-gaap	Other Comprehensive Income Loss Before Tax OtherComprehensiveIncomeLossBeforeTax	-59300000	usd
	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	33600000	usd
	us-gaap	Other Comprehensive Income Loss Tax OtherComprehensiveIncomeLossTax	-14700000	usd
	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	101600000	usd
	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-44600000	usd
	us-gaap	Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Transfers Net FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet	0	usd
CY2024Q3	us-gaap	Net Income Loss NetIncomeLoss	55500000	usd
CY2024Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.54
CY2023Q3	us-gaap	Net Income Loss NetIncomeLoss	72700000	usd
CY2023Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.70
	us-gaap	Net Income Loss NetIncomeLoss	173900000	usd
	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.69
	us-gaap	Net Income Loss NetIncomeLoss	196000000.0	usd
	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	1.89
CY2024Q3	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	19800000	usd
CY2023Q3	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	-100000	usd
	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	34100000	usd
	us-gaap	Provision For Loan And Lease Losses ProvisionForLoanAndLeaseLosses	26800000	usd
CY2024Q3	us-gaap	Assets Assets	29595500000	usd
CY2023Q4	us-gaap	Assets Assets	30671200000	usd
CY2024Q3	us-gaap	Investments Investments	8275600000	usd
CY2023Q4	us-gaap	Investments Investments	9049400000	usd
	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	102200000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Amortized Cost Allowance For Credit Loss Excluding Accrued Interest DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest	0	usd
CY2024Q3	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	700000	usd
CY2023Q4	us-gaap	Debt Securities Held To Maturity Allowance For Credit Loss DebtSecuritiesHeldToMaturityAllowanceForCreditLoss	800000	usd
CY2024Q3	us-gaap	Loans And Leases Receivable Deferred Income LoansAndLeasesReceivableDeferredIncome	11800000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Deferred Income LoansAndLeasesReceivableDeferredIncome	12500000	usd
CY2024Q3	us-gaap	Other Real Estate OtherRealEstate	4400000	usd
CY2023Q4	us-gaap	Other Real Estate OtherRealEstate	16500000	usd
CY2024Q3	us-gaap	Loans And Leases Receivable Allowance LoansAndLeasesReceivableAllowance	225400000	usd
CY2023Q4	us-gaap	Loans And Leases Receivable Allowance LoansAndLeasesReceivableAllowance	227700000	usd
CY2024Q3	us-gaap	Allowance For Doubtful Accounts Receivable Current AllowanceForDoubtfulAccountsReceivableCurrent	5600000	usd
CY2023Q4	us-gaap	Allowance For Doubtful Accounts Receivable Current AllowanceForDoubtfulAccountsReceivableCurrent	18400000	usd
CY2024Q2	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	5800000	usd
CY2023Q2	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	20800000	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
CY2024Q3	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	5600000	usd
CY2023Q3	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	18800000	usd
CY2024Q3	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	5600000	usd
CY2023Q3	us-gaap	Off Balance Sheet Credit Loss Liability OffBalanceSheetCreditLossLiability	18800000	usd
CY2024Q3	us-gaap	Liabilities Liabilities	26229700000	usd
CY2023Q4	us-gaap	Liabilities Liabilities	27443700000	usd
CY2024Q3	us-gaap	Deposits Deposits	22864100000	usd
CY2023Q4	us-gaap	Deposits Deposits	23323100000	usd
CY2024Q3	us-gaap	Noninterest Bearing Deposit Liabilities NoninterestBearingDepositLiabilities	5919000000	usd
CY2024Q3	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2080000000	usd
CY2023Q4	us-gaap	Other Short Term Borrowings OtherShortTermBorrowings	2603000000	usd
CY2024Q3	us-gaap	Stockholders Equity StockholdersEquity	3365800000	usd
CY2023Q4	us-gaap	Stockholders Equity StockholdersEquity	3227500000	usd

Files In Submission

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$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

2024 Q3 Form 10-Q Financial Statement

#000086041324000130 Filed on November 04, 2024

Income Statement

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Cashflow Statement

Facts In Submission

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$FIBK FIRST INTERSTATE BANCSYSTEM INC Stock $fibk

2024 Q3 Form 10-Q Financial Statement

#000086041324000130 Filed on November 04, 2024

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$FIBK
FIRST INTERSTATE BANCSYSTEM INC Stock $fibk