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Legal Proceedings (Details) (Pending Litigation, Kelly Logging Inc. v. First Interstate Bank, USD $)
In Thousands, unless otherwise specified
 0 Months Ended 1 Months Ended 0 Months Ended
Oct. 01, 2014
Aug. 31, 2014
Apr. 21, 2015
Mar. 31, 2015
Loss Contingencies [Line Items]        
Loss Contingency, Damages Awarded, Value $ 17,047us-gaap_LossContingencyDamagesAwardedValue $ 17,047us-gaap_LossContingencyDamagesAwardedValue    
Loss Contingency, Compensatory Damages Awarded, Value   287fibk_LossContingencyCompensatoryDamagesAwardedValue    
Loss Contingency, Punitive Damages Awarded, Value   16,760fibk_LossContingencyPunitiveDamagesAwardedValue    
Loss Contingency Accrual       4,000us-gaap_LossContingencyAccrualAtCarryingValue
Subsequent Event [Member]        
Loss Contingencies [Line Items]        
Awarded Plaintiff Attorney's Fees     7,500fibk_AwardedPlaintiffAttorneysFees
/ us-gaap_LitigationCaseAxis
= fibk_KellyLoggingInc.v.FirstInterstateBankMember
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		  
Awarded Plaintiff Costs     91fibk_AwardedPlaintiffCosts
/ us-gaap_LitigationCaseAxis
= fibk_KellyLoggingInc.v.FirstInterstateBankMember
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		  
Litigation Settlement, Amount     $ 24,638us-gaap_LitigationSettlementAmount
/ us-gaap_LitigationCaseAxis
= fibk_KellyLoggingInc.v.FirstInterstateBankMember
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember