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Company - Owned Life Insurance (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Schedule of Company - Owned Life Insurance, By Type    
Company owned life insurance $ 153,821us-gaap_LifeInsuranceCorporateOrBankOwnedAmount $ 122,175us-gaap_LifeInsuranceCorporateOrBankOwnedAmount
Key executive, principal shareholder    
Schedule of Company - Owned Life Insurance, By Type    
Cash surrender value of life insurance 3,798us-gaap_CashSurrenderValueOfLifeInsurance
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= fibk_KeyExecutivePrincipalShareholderMember		 3,660us-gaap_CashSurrenderValueOfLifeInsurance
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= fibk_KeyExecutivePrincipalShareholderMember
Key executive split dollar    
Schedule of Company - Owned Life Insurance, By Type    
Cash surrender value of life insurance 4,707us-gaap_CashSurrenderValueOfLifeInsurance
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= fibk_KeyExecutiveSplitDollarMember		 4,628us-gaap_CashSurrenderValueOfLifeInsurance
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= fibk_KeyExecutiveSplitDollarMember
Group life    
Schedule of Company - Owned Life Insurance, By Type    
Cash surrender value of life insurance 145,316us-gaap_CashSurrenderValueOfLifeInsurance
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= fibk_GroupLifeMember		  
FIB | Group life    
Schedule of Company - Owned Life Insurance, By Type    
Cash surrender value of life insurance 132,111us-gaap_CashSurrenderValueOfLifeInsurance
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= fibk_GroupLifeMember		 113,887us-gaap_CashSurrenderValueOfLifeInsurance
/ dei_LegalEntityAxis
= fibk_FirstInterstateBankMember
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= fibk_GroupLifeMember
Mountain West Bank | Group life    
Schedule of Company - Owned Life Insurance, By Type    
Cash surrender value of life insurance $ 13,205us-gaap_CashSurrenderValueOfLifeInsurance
/ dei_LegalEntityAxis
= fibk_MountainWestBankMember
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= fibk_GroupLifeMember