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Investment Securities - Maturities of Investment Securities (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity    
Available-for-sale amortized cost, within one year $ 252,677us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis  
Available-for-sale amortized cost, after one year but within five years 1,218,143us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis  
Available-for-sale amortized cost, after five years but within ten years 188,520us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis  
Available-for-sale amortized cost, after ten years 49,154us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenAmortizedCostBasis  
Available-for-Sale, Amortized Cost 1,708,494us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis 1,971,757us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity    
Available-for-sale estimated fair value, within one year 254,560us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue  
Available-for-sale estimated fair value, after one year but within five years 1,219,799us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue  
Available-for-sale estimated fair value, after ten years 188,015us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue  
Available-for-ale estimated fair value, after five years but within ten years 49,550us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenFairValue  
Available-for-Sale, Estimated Fair Value 1,711,924us-gaap_AvailableForSaleSecuritiesDebtSecurities 1,947,706us-gaap_AvailableForSaleSecuritiesDebtSecurities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount    
Held-to-maturity amortized cost, within one year 85,751us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount  
Held-to-maturity amoritzed cost, after one year but within five years 283,000us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount  
Held-to-maturity amortized cost, after five years but within ten years 150,601us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount  
Held-to-maturity amortized cost, after ten years 55,834us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount  
Held-to-Maturity, Amortized Cost 575,186us-gaap_HeldToMaturitySecurities 203,837us-gaap_HeldToMaturitySecurities
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity    
Held-to-maturity estimated fair value, within one year 86,983us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue  
Held-to-maturity estimated fair value, after one year but within five years 286,783us-gaap_HeldToMaturitySecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue  
Held-to-maturity estimated fair value, after five years but within ten years 153,287us-gaap_HeldToMaturitySecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue  
Held-to-maturity estimated fair value, after ten years 57,480us-gaap_HeldToMaturitySecuritiesDebtMaturitiesRollingAfterTenYearsFairValue  
Held-to-Maturity Estimated Fair Value 584,533us-gaap_HeldToMaturitySecuritiesFairValue 205,926us-gaap_HeldToMaturitySecuritiesFairValue
Available-for-sale Securities and Held-to-maturity Securities    
Securities sold under agreements to repurchase 1,351,787fibk_PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtCost 1,288,750fibk_PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtCost
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value 1,356,341us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtFairValue 1,278,663us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtFairValue
Callable within one year    
Available-for-sale Securities and Held-to-maturity Securities    
Investment securities primarily classified as available-for-sale, amortized costs, after one year but within five years 176,023fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis
/ fibk_InvestmentSecuritiesDescriptionAxis
= fibk_CallableWithinOneYearMember		  
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years 175,892fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveFairValue
/ fibk_InvestmentSecuritiesDescriptionAxis
= fibk_CallableWithinOneYearMember		  
Callable structured notes    
Available-for-sale Securities and Held-to-maturity Securities    
Investment securities primarily classified as available-for-sale, amortized costs, after one year but within five years 29,995fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis
/ fibk_InvestmentSecuritiesDescriptionAxis
= fibk_CallableStructuredNotesMember		  
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years 30,012fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveFairValue
/ fibk_InvestmentSecuritiesDescriptionAxis
= fibk_CallableStructuredNotesMember		  
Mortgage Backed Securities, Variable Rate    
Available-for-sale Securities and Held-to-maturity Securities    
Available-for-sale securities, amortized cost basis $ 40,666us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_DebtSecurityAxis
= fibk_MortgageBackedSecuritiesVariableRateMember		 $ 46,315us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_DebtSecurityAxis
= fibk_MortgageBackedSecuritiesVariableRateMember