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Investment Securities - Realized Gains (Losses) on Investments (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]      
Gross realized gains $ 274us-gaap_AvailableForSaleSecuritiesGrossRealizedGains $ 49us-gaap_AvailableForSaleSecuritiesGrossRealizedGains $ 351us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Gross realized losses 213us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses 48us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses 3us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Political Subdivisions      
Schedule of Available-for-sale Securities      
Available-for-sale Securities, Amortized Cost Basis 131,845us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		    
Political Subdivisions | Montana, Wyoming, and South Dakota      
Schedule of Available-for-sale Securities      
Available-for-sale Securities, Amortized Cost Basis $ 68,927us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_StatementGeographicalAxis
= fibk_MontanaWyomingAndSouthDakotaMember