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Subordinated Debentures Held by Subsidiary Trusts - Financial Insruments Subject to Mandatory Redemption (Details) (Trusts, USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Trust Preferred Securities Subject to Mandatory Redemption  
Financial Instruments Subject to Mandatory Redemption by Settlement Terms  
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Share Value, Amount $ 80,000us-gaap_SharesSubjectToMandatoryRedemptionSettlementTermsAmount
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= fibk_TrustsMember
/ us-gaap_ScheduleOfSharesSubjectToMandatoryRedemptionBySettlementTermsAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
Common Stock Subject to Mandatory Redemption  
Financial Instruments Subject to Mandatory Redemption by Settlement Terms  
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Share Value, Amount $ 2,477us-gaap_SharesSubjectToMandatoryRedemptionSettlementTermsAmount
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= fibk_TrustsMember
/ us-gaap_ScheduleOfSharesSubjectToMandatoryRedemptionBySettlementTermsAxis
= us-gaap_CommonStockSubjectToMandatoryRedemptionMember