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Schedule of Loans Renegotiated in Troubled Debt Restructurings (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
notes
Mar. 31, 2014
Dec. 31, 2014
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Loans Renegotiated in Troubled Debt Restructurings $ 39,645us-gaap_FinancingReceivableModificationsRecordedInvestment   $ 44,227us-gaap_FinancingReceivableModificationsRecordedInvestment
Loans Renegotiated in Troubled Debt Restructurings, Non-Accrual Loans 23,575fibk_FinancingReceivableModificationsRecordedInvestmentCashBasisMethod   23,275fibk_FinancingReceivableModificationsRecordedInvestmentCashBasisMethod
Loans Renegotiated in Troubled Debt Restructurings, Accrual Loans 16,070fibk_FinancingReceivableModificationsRecordedInvestmentAccrualMethod   20,952fibk_FinancingReceivableModificationsRecordedInvestmentAccrualMethod
Number of Notes 1us-gaap_FinancingReceivableModificationsNumberOfContracts2    
Financing Receivable, Modifications, Pre-Modification Recorded Investment 10us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2    
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down 0us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown 0us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown  
Loans and Leases Receivable, Impaired, Commitment to Lend 0us-gaap_LoansAndLeasesReceivableImpairedCommitmentToLend    
Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
   
Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 10us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
   
Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
   
Other      
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember
   
Commercial real estate      
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Financing Receivable, Modifications, Subsequent Default, Number of Contracts 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
   
Financing Receivable, Modifications, Subsequent Default, Recorded Investment 1,822us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
   
Commercial      
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Number of Notes 1us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
   
Financing Receivable, Modifications, Pre-Modification Recorded Investment 10us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
   
Commercial | Interest only period      
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestOnlyPeriodConcessionMember
   
Commercial | Extension of terms or maturity      
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 10us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_ExtensionOfTermsOrMaturityConcessionMember
   
Commercial | Interest rate adjustment      
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Financing Receivable, Modifications, Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_InterestRateAdjustmentConcessionMember
   
Commercial | Other      
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]      
Financing Receivable, Modifications, Pre-Modification Recorded Investment $ 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ fibk_FinancingReceivableTroubledDebtRestructuringConcessionTypeAxis
= fibk_OtherConcessionMember