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Acquisitions Business Acquisition, Pro Forma Information (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Business Acquisition [Line Items]    
Expenses Related to Acquisition Excluded From Pro Forma Net Income $ 5,052fibk_ExpensesRelatedtoAcquisitionExcludedFromProFormaNetIncome  
Business Acquisition, Pro Forma Interest Income 280,102fibk_BusinessAcquisitionProFormaInterestIncome 280,455fibk_BusinessAcquisitionProFormaInterestIncome
Business Acquisition, Pro Forma Interest Expense 114,984fibk_BusinessAcquisitionProFormaInterestExpense 120,005fibk_BusinessAcquisitionProFormaInterestExpense
Business Acquisition, Pro Forma Revenue 395,086us-gaap_BusinessAcquisitionsProFormaRevenue 400,460us-gaap_BusinessAcquisitionsProFormaRevenue
Business Acquisition, Pro Forma Net Income (Loss) $ 87,129us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss $ 84,264us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss