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Fair Value Measurements Fair Value Inputs, Quantitative Information (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Loans and Leases Receivable, Impaired Loans [Member] | Minimum [Member] | Market Approach Valuation Technique [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs, Discount Rate 0.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember		 0.00%us-gaap_FairValueInputsDiscountRate
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Loans and Leases Receivable, Impaired Loans [Member] | Maximum [Member] | Market Approach Valuation Technique [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs, Discount Rate (51.00%)us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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= us-gaap_MaximumMember
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= us-gaap_MarketApproachValuationTechniqueMember		 (51.00%)us-gaap_FairValueInputsDiscountRate
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Loans and Leases Receivable, Impaired Loans [Member] | Weighted Average [Member] | Market Approach Valuation Technique [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs, Discount Rate (19.00%)us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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= us-gaap_WeightedAverageMember
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Real Estate Acquired in Satisfaction of Debt [Member] | Minimum [Member] | Market Approach Valuation Technique [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs, Discount Rate 0.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember		 0.00%us-gaap_FairValueInputsDiscountRate
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Real Estate Acquired in Satisfaction of Debt [Member] | Maximum [Member] | Market Approach Valuation Technique [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs, Discount Rate (50.00%)us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember		 (50.00%)us-gaap_FairValueInputsDiscountRate
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Real Estate Acquired in Satisfaction of Debt [Member] | Weighted Average [Member] | Market Approach Valuation Technique [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs, Discount Rate (15.00%)us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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= us-gaap_WeightedAverageMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember		 (15.00%)us-gaap_FairValueInputsDiscountRate
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Asses to be Disposed of by Sale | Minimum [Member] | Market Approach Valuation Technique [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs, Discount Rate 0.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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= us-gaap_MinimumMember
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= us-gaap_MarketApproachValuationTechniqueMember		 0.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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Asses to be Disposed of by Sale | Maximum [Member] | Market Approach Valuation Technique [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs, Discount Rate (9.00%)us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember		 (9.00%)us-gaap_FairValueInputsDiscountRate
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Asses to be Disposed of by Sale | Weighted Average [Member] | Market Approach Valuation Technique [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs, Discount Rate (5.00%)us-gaap_FairValueInputsDiscountRate
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Estimated Fair Value | Fair Value Measured on a Non-recurring Basis    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Impaired loans 35,098fibk_LoansReceivableImpairedFairValueDisclosure
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= us-gaap_FairValueMeasurementsNonrecurringMember		 30,494fibk_LoansReceivableImpairedFairValueDisclosure
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Long-lived assets to be disposed of by sale 1,083us-gaap_AssetsHeldForSaleLongLivedFairValueDisclosure
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Other real estate owned 6,499fibk_RealEstateAcquiredThroughForeclosureFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember		 4,554fibk_RealEstateAcquiredThroughForeclosureFairValueDisclosure
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