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Mortgage Servicing Rights - At Amortized Cost (Details) (Mortgage loans, USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mortgage loans
     
Servicing Asset at Amortized Value, Balance [Roll Forward]      
Servicing asset at amortized cost, before valuation reserve, beginning $ 14,018fibk_ServicingAssetAtAmortizedCostBeforeValuationReserve
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
$ 13,224fibk_ServicingAssetAtAmortizedCostBeforeValuationReserve
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
$ 13,450fibk_ServicingAssetAtAmortizedCostBeforeValuationReserve
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Sales of mortgage servicing rights 0us-gaap_ServicingAssetAtAmortizedValueDisposals
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
0us-gaap_ServicingAssetAtAmortizedValueDisposals
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
(735)us-gaap_ServicingAssetAtAmortizedValueDisposals
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Originations of mortgage servicing rights 2,717fibk_ServicingAssetAtAmortizedValueOriginations
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
3,581fibk_ServicingAssetAtAmortizedValueOriginations
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
4,563fibk_ServicingAssetAtAmortizedValueOriginations
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Amorization expense (2,361)us-gaap_ServicingAssetAtAmortizedValueAmortization1
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
(2,787)us-gaap_ServicingAssetAtAmortizedValueAmortization1
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
(3,501)us-gaap_ServicingAssetAtAmortizedValueAmortization1
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Write-off of permanent impairment 0us-gaap_ServicingAssetAtAmortizedCostOtherThanTemporaryImpairments
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
0us-gaap_ServicingAssetAtAmortizedCostOtherThanTemporaryImpairments
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
(553)us-gaap_ServicingAssetAtAmortizedCostOtherThanTemporaryImpairments
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Servicing asset at amortized cost, before valuation reserve, ending 14,374fibk_ServicingAssetAtAmortizedCostBeforeValuationReserve
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
14,018fibk_ServicingAssetAtAmortizedCostBeforeValuationReserve
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
13,224fibk_ServicingAssetAtAmortizedCostBeforeValuationReserve
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Less valuation reserve (336)us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
(472)us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
(571)us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Servicing asset at fair value, ending 14,038us-gaap_ServicingAssetAtAmortizedValueFairValue
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
13,546us-gaap_ServicingAssetAtAmortizedValueFairValue
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
12,653us-gaap_ServicingAssetAtAmortizedValueFairValue
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Principal balance of serviced loans underlying mortgage servicing rights 2,615,311us-gaap_PrincipalAmountOutstandingOfLoansHeldInPortfolio
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
2,416,621us-gaap_PrincipalAmountOutstandingOfLoansHeldInPortfolio
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
2,146,351us-gaap_PrincipalAmountOutstandingOfLoansHeldInPortfolio
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Mortgage servicing rights as a percentage of serviced loans 0.0054fibk_ServicingAssetAsPercentageOfServicedLoansHeldInPortfolio
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
0.0056fibk_ServicingAssetAsPercentageOfServicedLoansHeldInPortfolio
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
0.0059fibk_ServicingAssetAsPercentageOfServicedLoansHeldInPortfolio
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
Disclosure of Transfer of Securitizations or Asset-backed Financing Financial Assets Accounted for as Sale      
Carrying value of sold mortgage servicing rights   735fibk_TransfersOfFinancialAssetsAccountedForAsSaleCarryingValue
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
 
Losses on sale of mortgage servicing rights recorded as other expense   $ (19)us-gaap_SecuritizationFinancialAssetForWhichTransferIsAccountedAsSaleGainLossOnSale
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember