XML 36 R77.htm IDEA: XBRL DOCUMENT v2.4.1.9
Loans - Schedule of Loans Renegotiated in Troubled Debt Restructurings (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
notes
Dec. 31, 2013
notes
Dec. 31, 2012
notes
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down $ 0us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown $ 0us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown $ 0us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown
Loans renegotiated in troubled debt restructurings 44,227us-gaap_FinancingReceivableModificationsRecordedInvestment 59,792us-gaap_FinancingReceivableModificationsRecordedInvestment  
Loans renegotiated in troubled debt restructurings, non-accrual loans 23,275fibk_FinancingReceivableModificationsRecordedInvestmentNonAccrual 38,011fibk_FinancingReceivableModificationsRecordedInvestmentNonAccrual  
Loans tenegotiated in troubled debt restructurings, accrual loans 20,952fibk_FinancingReceivableModificationsRecordedInvestmentAccrualMethod 21,781fibk_FinancingReceivableModificationsRecordedInvestmentAccrualMethod  
Financing receivable, modifications, subsequent default, number of contracts, during period 20us-gaap_FinancingReceivableModificationsNumberOfContracts2 43us-gaap_FinancingReceivableModificationsNumberOfContracts2 38us-gaap_FinancingReceivableModificationsNumberOfContracts2
Financing Receivable, Modifications, Pre-Modification Recorded Investment 7,190us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 28,403us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 22,760us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 5,229us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
1,684us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
955us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 785us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
9,980us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
2,704us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 84us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
14,263us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
6,494us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
Other      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 1,092us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
2,476us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
12,607us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
Commercial real estate      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing receivable, modifications, subsequent default, number of contracts, during period 13us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
19us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
16us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
Financing Receivable, Modifications, Pre-Modification Recorded Investment 6,556us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
15,651us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
14,074us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
Commercial real estate | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 4,753us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
543us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Commercial real estate | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 672us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
1,378us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
959us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
Commercial real estate | Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 84us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
11,420us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
4,504us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
Commercial real estate | Other      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 1,047us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
2,310us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
8,611us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
Commercial construction real estate      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing receivable, modifications, subsequent default, number of contracts, during period     1us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Financing Receivable, Modifications, Pre-Modification Recorded Investment     3,155us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Commercial construction real estate | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment     0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Commercial construction real estate | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment     0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
Commercial construction real estate | Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment     0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
Commercial construction real estate | Other      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment     3,155us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
Land acquisition & development construction real estate      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing receivable, modifications, subsequent default, number of contracts, during period   8us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
5us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
Financing Receivable, Modifications, Pre-Modification Recorded Investment   9,788us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
3,380us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
Land acquisition & development construction real estate | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   528us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Land acquisition & development construction real estate | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   7,308us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
1,000us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
Land acquisition & development construction real estate | Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   1,952us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
1,757us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
Land acquisition & development construction real estate | Other      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
623us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
Residential construction real estate      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing receivable, modifications, subsequent default, number of contracts, during period   3us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
2us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
Financing Receivable, Modifications, Pre-Modification Recorded Investment   819us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
513us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
Residential construction real estate | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Residential construction real estate | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   408us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
280us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
Residential construction real estate | Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   411us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
233us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
Residential construction real estate | Other      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialConstructionRealEstateMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
Total construction loans      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing receivable, modifications, subsequent default, number of contracts, during period   11us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
8us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
Financing Receivable, Modifications, Pre-Modification Recorded Investment   10,607us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
7,048us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
Total construction loans | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   528us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Total construction loans | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   7,716us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
1,280us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
Total construction loans | Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   2,363us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
1,990us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
Total construction loans | Other      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
3,778us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
Residential real estate      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing receivable, modifications, subsequent default, number of contracts, during period 1us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
5us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
2us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Financing Receivable, Modifications, Pre-Modification Recorded Investment, Restructured During Period 15fibk_FinancingReceivableModificationsPreModificationRecordedInvestmentRestructuredDuringPeriod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
   
Financing Receivable, Modifications, Pre-Modification Recorded Investment   787us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
593us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Residential real estate | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment, Restructured During Period 0fibk_FinancingReceivableModificationsPreModificationRecordedInvestmentRestructuredDuringPeriod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
   
Financing Receivable, Modifications, Pre-Modification Recorded Investment   0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
568us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Residential real estate | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment, Restructured During Period 0fibk_FinancingReceivableModificationsPreModificationRecordedInvestmentRestructuredDuringPeriod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
   
Financing Receivable, Modifications, Pre-Modification Recorded Investment   708us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
25us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
Residential real estate | Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment, Restructured During Period 0fibk_FinancingReceivableModificationsPreModificationRecordedInvestmentRestructuredDuringPeriod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
   
Financing Receivable, Modifications, Pre-Modification Recorded Investment   0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
Residential real estate | Other      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment, Restructured During Period 15fibk_FinancingReceivableModificationsPreModificationRecordedInvestmentRestructuredDuringPeriod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
   
Financing Receivable, Modifications, Pre-Modification Recorded Investment   79us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
Agricultural real estate      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing receivable, modifications, subsequent default, number of contracts, during period   1us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
1us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
Financing Receivable, Modifications, Pre-Modification Recorded Investment   188us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
154us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
Agricultural real estate | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Agricultural real estate | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
154us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
Agricultural real estate | Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   188us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
Agricultural real estate | Other      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment   0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
Total real estate loans      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing receivable, modifications, subsequent default, number of contracts, during period 14us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
36us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
27us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
Financing Receivable, Modifications, Pre-Modification Recorded Investment 6,571us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
27,233us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
21,869us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
Total real estate loans | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 4,753us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
1,071us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
568us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Total real estate loans | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 672us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
9,802us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
2,418us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
Total real estate loans | Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 84us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
13,971us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
6,494us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
Total real estate loans | Other      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 1,062us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
2,389us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
12,389us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
Total consumer loans      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing receivable, modifications, subsequent default, number of contracts, during period 1us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
1us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
1us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
Financing Receivable, Modifications, Pre-Modification Recorded Investment 113us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
27us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
69us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
Total consumer loans | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Total consumer loans | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 113us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
69us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
Total consumer loans | Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
27us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
Total consumer loans | Other      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
Commercial      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing receivable, modifications, subsequent default, number of contracts, during period 5us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
6us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
10us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
Financing Receivable, Modifications, Pre-Modification Recorded Investment 506us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
1,143us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
822us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
Commercial | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 476us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
613us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
387us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
Commercial | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
178us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
217us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
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