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Loans - Schedule of Loans by Class (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Loans and Leases Receivable    
Loans held for investment $ 4,856,615fibk_LoansAndLeasesReceivableHeldForInvestment $ 4,303,992fibk_LoansAndLeasesReceivableHeldForInvestment
Mortgage loans held for sale 40,828us-gaap_LoansHeldForSaleMortgages 40,861us-gaap_LoansHeldForSaleMortgages
Total loans 4,897,443us-gaap_LoansAndLeasesReceivableGrossCarryingAmount 4,344,853us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Commercial real estate    
Loans and Leases Receivable    
Loans held for investment 1,639,422fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember		 1,449,174fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
Land acquisition & development construction real estate    
Loans and Leases Receivable    
Loans held for investment 220,443fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember		 205,911fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
Residential construction real estate    
Loans and Leases Receivable    
Loans held for investment 96,580fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember		 76,488fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
Commercial construction real estate    
Loans and Leases Receivable    
Loans held for investment 101,246fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 69,236fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Total construction loans    
Loans and Leases Receivable    
Loans held for investment 418,269fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember		 351,635fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
Residential real estate    
Loans and Leases Receivable    
Loans held for investment 999,903fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 867,912fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Loans receivable, maturity 15 years  
Agricultural real estate    
Loans and Leases Receivable    
Loans held for investment 167,659fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember		 173,534fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
Total real estate loans    
Loans and Leases Receivable    
Loans held for investment 3,225,253fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember		 2,842,255fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
Indirect consumer    
Loans and Leases Receivable    
Loans held for investment 552,863fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember		 476,012fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
Other consumer    
Loans and Leases Receivable    
Loans held for investment 144,141fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember		 133,039fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
Credit card consumer    
Loans and Leases Receivable    
Loans held for investment 65,467fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember		 62,536fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
Total consumer loans    
Loans and Leases Receivable    
Loans held for investment 762,471fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember		 671,587fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
Commercial    
Loans and Leases Receivable    
Loans held for investment 740,073fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember		 676,544fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
Agricultural    
Loans and Leases Receivable    
Loans held for investment 124,859fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember		 111,872fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
Other, including overdrafts    
Loans and Leases Receivable    
Loans held for investment 3,959fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherFinancingReceivableIncludingOverdraftsMember		 1,734fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherFinancingReceivableIncludingOverdraftsMember
Home equity line of credit    
Loans and Leases Receivable    
Loans held for investment $ 298,692fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember		 $ 272,415fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
Loans receivable, loan to value limit 80.00%fibk_LoansReceivableLoanToValueLimit
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember		  
Commercial and agricultural real estate    
Loans and Leases Receivable    
Loans receivable, maturity 5 years