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Subsequent Events Subsequent Events (Details) (Common Class A Stoc, USD $)
Jan. 22, 2015
Subsequent Event [Member]  
Subsequent Event [Line Items]  
Dividends Payable, Amount Per Share $ 0.20us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_DividendDeclaredMember
January 2015 Board Approved Stock Repurchased Program [Member]  
Subsequent Event [Line Items]  
Additional Shares Authorized 1,000,000us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
/ us-gaap_SubsequentEventTypeAxis
= fibk_January2015BoardApprovedStockRepurchasedProgramMember