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Maturities of Investment Securities (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity    
Available-for-Sale Amortized Cost, Within One Year $ 288,960us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis  
Available-for-Sale Amortized Cost, After One Year but Within Five Years 1,316,177us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis  
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years 126,241us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis  
Available-for-Sale Amortized Cost, After Ten Years 31,641us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenAmortizedCostBasis  
Available-for-Sale, Amortized Cost 1,763,019us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis 1,708,494us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity    
Available-for-Sale Estimated Fair Value, Within One Year 292,432us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue  
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years 1,325,048us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue  
Available-for-Sale Estimated Fair Value, After Ten Years 127,514us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue  
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years 32,063us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenFairValue  
Available-for-Sale, Estimated Fair Value 1,777,057us-gaap_AvailableForSaleSecuritiesDebtSecurities 1,711,924us-gaap_AvailableForSaleSecuritiesDebtSecurities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount    
Held-to-Maturity Amortized Cost, Within One Year 79,106us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount  
Held-to-Maturity Amortized Cost, After One Year but Within Five Years 286,575us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount  
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years 133,958us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount  
Held-to-Maturity Amortized Cost, After Ten Years 64,208us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount  
Held-to-Maturity, Amortized Cost 563,847us-gaap_HeldToMaturitySecurities 575,186us-gaap_HeldToMaturitySecurities
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity    
Held-to-Maturity Estimated Fair Value, Within One Year 80,955us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue  
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years 292,101us-gaap_HeldToMaturitySecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue  
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years 137,489us-gaap_HeldToMaturitySecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue  
Held-to-Maturity Estimated Fair Value, After Ten Years 66,386us-gaap_HeldToMaturitySecuritiesDebtMaturitiesRollingAfterTenYearsFairValue  
Held-to-Maturity, Estimated Fair Value 576,931us-gaap_HeldToMaturitySecuritiesFairValue 584,533us-gaap_HeldToMaturitySecuritiesFairValue
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 166,196fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis
/ fibk_InvestmentSecuritiesDescriptionAxis
= fibk_CallableWithinOneYearMember		  
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years 166,636fibk_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 29,996fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis
/ fibk_InvestmentSecuritiesDescriptionAxis
= fibk_CallableStructuredNotesMember		  
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years $ 30,030fibk_InvestmentSecuritiesPrimarilyClassifiedAsAvailableForSaleDebtMaturitiesRollingYearTwoThroughFiveFairValue
/ fibk_InvestmentSecuritiesDescriptionAxis
= fibk_CallableStructuredNotesMember