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OREO (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]    
Mortgage Loans in Process of Foreclosure, Amount $ 0us-gaap_MortgageLoansInProcessOfForeclosureAmount  
Real Estate Acquired Through Foreclosure [Roll Forward]    
Beginning balance 13,554us-gaap_RealEstateAcquiredThroughForeclosure 15,504us-gaap_RealEstateAcquiredThroughForeclosure
Additions 3,257fibk_RealEstateAcquiredThroughForeclosureAdditions 3,415fibk_RealEstateAcquiredThroughForeclosureAdditions
Valuation adjustments (106)fibk_RealEstateAcquiredThroughForeclosureValuationAdjustments 0fibk_RealEstateAcquiredThroughForeclosureValuationAdjustments
Dispositions (1,571)fibk_RealEstateAcquiredThroughForeclosureDisposals (2,325)fibk_RealEstateAcquiredThroughForeclosureDisposals
Ending balance 15,134us-gaap_RealEstateAcquiredThroughForeclosure 16,594us-gaap_RealEstateAcquiredThroughForeclosure
Residential real estate    
Real Estate Acquired Through Foreclosure [Roll Forward]    
Ending balance $ 4,686us-gaap_RealEstateAcquiredThroughForeclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember