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Long-Term Debt and Other Borrowed Funds (Details) (USD $)
In Thousands, unless otherwise specified
 1 Months Ended
Feb. 29, 2008
Dec. 31, 2014
Dec. 31, 2013
Jan. 10, 2008
Feb. 28, 2008
Debt Instrument          
Long-term debt and capital lease obligations   $ 38,067us-gaap_LongTermDebtAndCapitalLeaseObligations $ 36,917us-gaap_LongTermDebtAndCapitalLeaseObligations    
Fiscal Year Maturities of Long-term Debt          
2015   285us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths      
2016   65us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo      
2017   71us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree      
2018   35,077us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour      
2019   83us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive      
Thereafter   2,486us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive      
Long-term debt and capital lease obligations   38,067us-gaap_LongTermDebtAndCapitalLeaseObligations 36,917us-gaap_LongTermDebtAndCapitalLeaseObligations    
Other borrowings   9us-gaap_OtherShortTermBorrowings 3us-gaap_OtherShortTermBorrowings    
Parent Company          
Debt Instrument          
Long-term debt   20,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 20,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		    
Subordinated Debt | Subordinated Term Loan, 6.81% | Parent Company          
Debt Instrument          
Long-term debt   20,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= fibk_SubordinatedTermLoanSixPointEightOnePercentMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember		 20,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= fibk_SubordinatedTermLoanSixPointEightOnePercentMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember		    
Fiscal Year Maturities of Long-term Debt          
Debt instrument, face amount       20,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= fibk_SubordinatedTermLoanSixPointEightOnePercentMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember		  
Debt instrument, interest rate, stated percentage   6.81%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= fibk_SubordinatedTermLoanSixPointEightOnePercentMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember		   6.81%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= fibk_SubordinatedTermLoanSixPointEightOnePercentMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember		  
Subordinated Debt | Subordinated Term Loan, Variable Rate | Subsidiaries          
Debt Instrument          
Long-term debt   15,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= fibk_SubordinatedTermLoanVariableRateMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember		 15,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= fibk_SubordinatedTermLoanVariableRateMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember		    
Fiscal Year Maturities of Long-term Debt          
Debt instrument, face amount         15,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= fibk_SubordinatedTermLoanVariableRateMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
Debt instrument, interest rate at period end   2.24%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_DebtInstrumentAxis
= fibk_SubordinatedTermLoanVariableRateMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember		      
Subordinated Debt | Subordinated Term Loan, Variable Rate | Subsidiaries | LIBOR          
Fiscal Year Maturities of Long-term Debt          
Debt instrument, basis spread on variable rate 2.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= fibk_SubordinatedTermLoanVariableRateMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember		        
Subordinated Debt | Note Payable, 6.24% | Parent Company          
Fiscal Year Maturities of Long-term Debt          
Debt instrument, interest rate, stated percentage       6.24%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= fibk_NotePayableSixPointTwoFourPercentMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember		  
Notes Payable to Banks | Notes Payable to FHLB | Subsidiaries          
Debt Instrument          
Long-term debt   225us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= fibk_NotesPayableToFhlbMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableToBanksMember		 225us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= fibk_NotesPayableToFhlbMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableToBanksMember		    
Fiscal Year Maturities of Long-term Debt          
FHLB, advances, weighted average interest rate   4.86%us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= fibk_NotesPayableToFhlbMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableToBanksMember		      
Capital Lease Obligations | Capital Lease Obligation, 8.00% | Subsidiaries          
Debt Instrument          
Capital Lease Obligations   1,643us-gaap_CapitalLeaseObligations
/ us-gaap_DebtInstrumentAxis
= fibk_CapitalLeaseObligationEightPercentMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CapitalLeaseObligationsMember		 1,692us-gaap_CapitalLeaseObligations
/ us-gaap_DebtInstrumentAxis
= fibk_CapitalLeaseObligationEightPercentMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CapitalLeaseObligationsMember		    
Fiscal Year Maturities of Long-term Debt          
Debt instrument, interest rate, stated percentage   8.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= fibk_CapitalLeaseObligationEightPercentMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CapitalLeaseObligationsMember		      
Federal Home Loan Bank | Notes Payable to FHLB | Subsidiaries          
Fiscal Year Maturities of Long-term Debt          
FHLB, advances, maximum amount available   701,768us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresMaximumAmountAvailable
/ us-gaap_DebtInstrumentAxis
= fibk_NotesPayableToFhlbMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_FederalHomeLoanBankAdvancesMember		      
Long-term FHLB advances     225us-gaap_FederalHomeLoanBankAdvancesLongTerm
/ us-gaap_DebtInstrumentAxis
= fibk_NotesPayableToFhlbMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_FederalHomeLoanBankAdvancesMember		    
Federal Funds Purchased          
Fiscal Year Maturities of Long-term Debt          
Line of credit facility, maximum borrowing capacity   115,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_FederalFundsPurchasedMember		      
Federal Reserve Bank Advances          
Fiscal Year Maturities of Long-term Debt          
Line of credit facility, maximum borrowing capacity   364,205us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_FederalReserveBankAdvancesMember		      
Notes Payable, Other Payables [Member] | Note Payable, 6.24% | Subsidiaries          
Fiscal Year Maturities of Long-term Debt          
Debt instrument, face amount   $ 1,199us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= fibk_NotePayableSixPointTwoFourPercentMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember