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Financial Instruments with Off-Balance Sheet Risk (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Entity Information    
Credit Extension Commitments $ 1,415,119fibk_CreditExtensionCommitments $ 1,238,269fibk_CreditExtensionCommitments
Credit Extension Commitments, Beyond One Year 363,609fibk_CreditExtensionCommitmentsBeyondOneYear 304,789fibk_CreditExtensionCommitmentsBeyondOneYear
Standby Letters of Credit    
Entity Information    
Credit Extension Commitments 58,950fibk_CreditExtensionCommitments
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialStandbyLetterOfCreditMember		 53,508fibk_CreditExtensionCommitments
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialStandbyLetterOfCreditMember
Unused Credit Card Lines    
Entity Information    
Credit Extension Commitments $ 476,752fibk_CreditExtensionCommitments
/ us-gaap_GuaranteeObligationsByNatureAxis
= fibk_UnusedCreditCardLinesMember		 $ 401,021fibk_CreditExtensionCommitments
/ us-gaap_GuaranteeObligationsByNatureAxis
= fibk_UnusedCreditCardLinesMember