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Schedule of Loans by Class (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Loans and Leases Receivable [Line Items]      
Loans held for investment $ 4,871,548fibk_LoansAndLeasesReceivableHeldForInvestment $ 4,856,615fibk_LoansAndLeasesReceivableHeldForInvestment  
Mortgage loans held for sale 55,758us-gaap_LoansReceivableHeldForSaleNetNotPartOfDisposalGroupMortgage 40,828us-gaap_LoansReceivableHeldForSaleNetNotPartOfDisposalGroupMortgage  
Total loans 4,927,306us-gaap_LoansAndLeasesReceivableGrossCarryingAmount 4,897,443us-gaap_LoansAndLeasesReceivableGrossCarryingAmount 4,364,838us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Commercial real estate      
Loans and Leases Receivable [Line Items]      
Loans held for investment 1,670,829fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
1,639,422fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Land acquisition & development construction real estate      
Loans and Leases Receivable [Line Items]      
Loans held for investment 209,033fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
220,443fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
 
Residential construction real estate      
Loans and Leases Receivable [Line Items]      
Loans held for investment 101,689fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
96,580fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
 
Commercial construction real estate      
Loans and Leases Receivable [Line Items]      
Loans held for investment 95,583fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialRealEstateConstructionMember
101,246fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialRealEstateConstructionMember
 
Total construction loans      
Loans and Leases Receivable [Line Items]      
Loans held for investment 406,305fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
418,269fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
 
Residential real estate      
Loans and Leases Receivable [Line Items]      
Loans held for investment 997,123fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
999,903fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
 
Agricultural real estate      
Loans and Leases Receivable [Line Items]      
Loans held for investment 156,734fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
167,659fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
 
Total real estate loans      
Loans and Leases Receivable [Line Items]      
Loans held for investment 3,230,991fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
3,225,253fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
 
Indirect consumer      
Loans and Leases Receivable [Line Items]      
Loans held for investment 566,225fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
552,863fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
 
Other consumer      
Loans and Leases Receivable [Line Items]      
Loans held for investment 140,529fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
144,141fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
 
Credit card consumer      
Loans and Leases Receivable [Line Items]      
Loans held for investment 61,708fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CreditCardReceivablesMember
65,467fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CreditCardReceivablesMember
 
Total consumer loans      
Loans and Leases Receivable [Line Items]      
Loans held for investment 768,462fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
762,471fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
 
Total loans 768,462us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
  670,406us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Commercial      
Loans and Leases Receivable [Line Items]      
Loans held for investment 754,149fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
740,073fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
 
Total loans 754,149us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
  707,237us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Agricultural      
Loans and Leases Receivable [Line Items]      
Loans held for investment 117,569fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
124,859fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
 
Other, including overdrafts      
Loans and Leases Receivable [Line Items]      
Loans held for investment $ 377fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherFinancingReceivableIncludingOverdraftsMember
$ 3,959fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherFinancingReceivableIncludingOverdraftsMember