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Mortgage Servicing Rights - At Fair Value (Details) (Mortgage loans, USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]  
Mortagage servicing rights, fair value $ 21,434us-gaap_ServicingAssetAtFairValueAmount
Servicing asset at fair value, assumptions used to estimate fair value, weighted average life 6 years 10 months 24 days
Minimum
  
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]  
Servicing assets at fair value, assumptions used to estimate fair value, discount rate 9.50%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed 0.60%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum
  
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]  
Servicing assets at fair value, assumptions used to estimate fair value, discount rate 21.00%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed 2.40%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed
/ us-gaap_FinancialInstrumentAxis
= fibk_MortgageLoansMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember