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Financial Instruments with Off-Balance Sheet Risk (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Entity Information [Line Items]  
Credit Extension Commitments $ 1,495,739fibk_CreditExtensionCommitments
Credit Extension Commitments, Beyond One Year 366,214fibk_CreditExtensionCommitmentsBeyondOneYear
Standby Letters of Credit  
Entity Information [Line Items]  
Credit Extension Commitments 59,119fibk_CreditExtensionCommitments
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialStandbyLetterOfCreditMember
Unused Credit Card Lines  
Entity Information [Line Items]  
Credit Extension Commitments $ 501,302fibk_CreditExtensionCommitments
/ us-gaap_GuaranteeObligationsByNatureAxis
= fibk_UnusedCreditCardLinesMember