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Loans - Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment $ 97,655us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment $ 123,376us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment $ 185,281us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Income Recognized 1,083us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod 1,692us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod 1,631us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income      
Interest income on impaired loans if interest had been accrued 4,951us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod 5,786us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod 8,463us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Commercial real estate      
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment 54,701us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember		 66,330us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember		 78,670us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
Income Recognized 876us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember		 1,092us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember		 1,339us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
Land acquisition & development construction real estate      
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment 13,056us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember		 19,523us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember		 44,457us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
Income Recognized 43us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember		 487us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember		 110us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_LandAcquisitionDevelopmentConstructionFinancingReceivableMember
Residential construction real estate      
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment 822us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember		 1,893us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember		 8,431us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
Income Recognized 0us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember		 0us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember		 4us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_ResidentialRealEstateConstructionFinancingReceivableMember
Commercial construction real estate      
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment 1,529us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 3,936us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 16,401us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Income Recognized 8us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 4us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 0us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Total construction loans      
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment 15,407us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember		 25,352us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember		 69,289us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
Income Recognized 51us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember		 491us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember		 114us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateConstructionFinancingReceivableMember
Residential real estate      
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment 4,537us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 8,104us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 13,703us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Income Recognized 5us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 17us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 26us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Agricultural real estate      
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment 8,774us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember		 8,230us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember		 6,936us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
Income Recognized 78us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember		 8us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember		 41us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalRealEstateFinancingReceivableMember
Total real estate loans      
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment 83,419us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember		 108,016us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember		 168,598us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
Income Recognized 1,010us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember		 1,608us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember		 1,520us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstateFinancingReceivableMember
Commercial      
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment 13,789us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember		 15,047us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember		 15,741us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
Income Recognized 50us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember		 68us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember		 84us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_CommercialFinancingReceivableMember
Agricultural      
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans      
Average Recorded Investment 447us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember		 313us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember		 942us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember
Income Recognized $ 23us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember		 $ 16us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember		 $ 27us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturalFinancingReceivableMember