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Mortgage Servicing Rights - Valuation of Impairment (Details) (Mortgage loans, USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mortgage loans
      
Valuation Allowance for Impairment of Recognized Servicing Assets      
Impairment reversals $ 136us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsRecoveries
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember		 $ 99us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsRecoveries
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember		 $ 771us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsRecoveries
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Permanent impairment charged against carrying value 0us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsAggregateWriteDowns
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember		 0us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsAggregateWriteDowns
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember		 553us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsAggregateWriteDowns
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Carrying value of sold mortgage servicing rights   735fibk_TransfersOfFinancialAssetsAccountedForAsSaleCarryingValue
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember		  
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale, Gain (Loss) on Sale   $ 19us-gaap_SecuritizationFinancialAssetForWhichTransferIsAccountedAsSaleGainLossOnSale
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember