XML 41 R45.htm IDEA: XBRL DOCUMENT v2.4.1.9
Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment $ 4,871,548fibk_LoansAndLeasesReceivableHeldForInvestment $ 4,856,615fibk_LoansAndLeasesReceivableHeldForInvestment
Commercial real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [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 Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 768,462fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 762,471fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Commercial    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 754,149fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember		 740,073fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Agricultural    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 117,569fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember		 124,859fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
Other Assets Especially Mentioned    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 140,492fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 154,084fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Commercial real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 76,271fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 84,533fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Land acquisition & development construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 10,929fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 11,826fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Residential construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 1,245fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 2,029fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Commercial construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 0fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialRealEstateConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 39fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialRealEstateConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Total construction loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 12,174fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 13,894fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Residential real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 9,374fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 10,473fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Agricultural real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 10,094fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 10,122fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Total real estate loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 107,913fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 119,022fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Indirect consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 832fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 916fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Other consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 538fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 553fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Credit card consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 0fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CreditCardReceivablesMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 0fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CreditCardReceivablesMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Total consumer loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 1,370fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 1,469fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Commercial    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 28,390fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 25,766fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Other Assets Especially Mentioned | Agricultural    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 2,819fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 7,827fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Substandard    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 156,887fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 163,675fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Commercial real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 82,814fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 83,448fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Land acquisition & development construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 14,475fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 15,016fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Residential construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 2,797fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 2,666fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Commercial construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 249fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialRealEstateConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 253fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialRealEstateConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Total construction loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 17,521fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 17,935fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Residential real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 8,381fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 10,848fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Agricultural real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 14,712fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 12,328fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Total real estate loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 123,428fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 124,559fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Indirect consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 1,415fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 1,590fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Other consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 877fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 1,085fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Credit card consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 416fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CreditCardReceivablesMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 348fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CreditCardReceivablesMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Total consumer loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 2,708fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 3,023fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Commercial    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 24,008fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 32,433fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard | Agricultural    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 6,743fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 3,660fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Doubtful    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 37,476fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 34,854fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Commercial real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 12,896fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 15,246fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Land acquisition & development construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 2,417fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 2,507fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Residential construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 0fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 0fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Commercial construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 3,164fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialRealEstateConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 2,442fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialRealEstateConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Total construction loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 5,581fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 4,949fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Residential real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 1,027fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 1,121fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Agricultural real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 613fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 612fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Total real estate loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 20,117fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 21,928fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Indirect consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 179fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 121fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Other consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 389fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 432fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Credit card consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 1,179fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CreditCardReceivablesMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 1,263fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CreditCardReceivablesMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Total consumer loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 1,747fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 1,816fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Commercial    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 14,877fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 10,273fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful | Agricultural    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 735fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 837fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Total Criticized Loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 334,855fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 352,613fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Commercial real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 171,981fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 183,227fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Land acquisition & development construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 27,821fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 29,349fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Residential construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 4,042fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 4,695fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Commercial construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 3,413fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialRealEstateConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 2,734fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialRealEstateConstructionMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Total construction loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 35,276fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 36,778fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Residential real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 18,782fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 22,442fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Agricultural real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 25,419fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 23,062fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Total real estate loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 251,458fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 265,509fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Indirect consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 2,426fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 2,627fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Other consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 1,804fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 2,070fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Credit card consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 1,595fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CreditCardReceivablesMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 1,611fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CreditCardReceivablesMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Total consumer loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 5,825fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 6,308fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Commercial    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment 67,275fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 68,472fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans | Agricultural    
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]    
Loans held for investment $ 10,297fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 $ 12,324fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember