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Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment $ 4,856,615fibk_LoansAndLeasesReceivableHeldForInvestment $ 4,303,992fibk_LoansAndLeasesReceivableHeldForInvestment
Commercial real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
Loans held for investment 999,903fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 867,912fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Agricultural real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
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 Recorded Investment, Criticized Loans    
Loans held for investment 124,859fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember		 111,872fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
Special Mention [Member]    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 154,084fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 159,081fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Commercial real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 84,533fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 79,747fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Land acquisition & development construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 11,826fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 13,211fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Residential construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 2,029fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 1,859fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Commercial construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 39fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 0fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Total construction loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 13,894fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 15,070fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Residential real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 10,473fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 7,500fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Agricultural real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 10,122fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 13,597fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Total real estate loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 119,022fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 115,914fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Indirect consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 916fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 875fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Other consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 553fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 573fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Credit card consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 0fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 0fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Total consumer loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 1,469fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 1,448fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Commercial    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 25,766fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 33,318fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Agricultural    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 7,827fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 8,401fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Substandard [Member]    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 163,675fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 154,100fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Commercial real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 83,448fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 86,426fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Land acquisition & development construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 15,016fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 19,677fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Residential construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 2,666fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 1,649fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Commercial construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 253fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 409fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Total construction loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 17,935fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 21,735fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Residential real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 10,848fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 7,188fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Agricultural real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 12,328fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 10,245fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Total real estate loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 124,559fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 125,594fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Indirect consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 1,590fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 1,524fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Other consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 1,085fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 969fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Credit card consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 348fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 392fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Total consumer loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 3,023fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 2,885fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Commercial    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 32,433fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 23,833fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Agricultural    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 3,660fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 1,788fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Doubtful [Member]    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 34,854fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 45,308fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Commercial real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 15,246fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 24,840fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Land acquisition & development construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 2,507fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 7,329fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Residential construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 0fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 277fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Commercial construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 2,442fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 84fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Total construction loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 4,949fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 7,690fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Residential real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 1,121fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 4,184fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Agricultural real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 612fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 2,370fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Total real estate loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 21,928fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 39,084fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Indirect consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 121fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 115fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Other consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 432fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 268fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Credit card consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 1,263fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 2,010fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Total consumer loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 1,816fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 2,393fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Commercial    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 10,273fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 3,745fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Agricultural    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 837fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 86fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Total Criticized Loans [Member]    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 352,613fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 358,489fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Commercial real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 183,227fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 191,013fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Land acquisition & development construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 29,349fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 40,217fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Residential construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 4,695fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 3,785fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Commercial construction real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 2,734fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 493fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Total construction loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 36,778fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 44,495fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Residential real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 22,442fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 18,872fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Agricultural real estate    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 23,062fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 26,212fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Total real estate loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 265,509fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 280,592fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Indirect consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 2,627fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 2,514fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerIndirectFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Other consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 2,070fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 1,810fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Credit card consumer    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 1,611fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 2,402fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Total consumer loans    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 6,308fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 6,726fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ConsumerFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Commercial    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment 68,472fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 60,896fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember
Total Criticized Loans [Member] | Agricultural    
Loans and Leases Receivable Recorded Investment, Criticized Loans    
Loans held for investment $ 12,324fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember		 $ 10,275fibk_LoansAndLeasesReceivableHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= fibk_TotalCriticizedLoansMember