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Commitments and Contingencies Legal Proceedings (Details) (Pending Litigation [Member], Kelly Logging Inc. v. First Interstate Bank [Member], USD $)
In Thousands, unless otherwise specified
 0 Months Ended 1 Months Ended
Oct. 01, 2014
Aug. 31, 2014
Dec. 31, 2014
Pending Litigation [Member] | Kelly Logging Inc. v. First Interstate Bank [Member]
      
Loss Contingencies      
Loss Contingency, Damages Awarded, Value $ 17,047us-gaap_LossContingencyDamagesAwardedValue
/ us-gaap_LitigationCaseAxis
= fibk_KellyLoggingInc.v.FirstInterstateBankMember
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember		 $ 17,047us-gaap_LossContingencyDamagesAwardedValue
/ us-gaap_LitigationCaseAxis
= fibk_KellyLoggingInc.v.FirstInterstateBankMember
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember		  
Loss Contingency, Compensatory Damages Awarded, Value   287fibk_LossContingencyCompensatoryDamagesAwardedValue
/ us-gaap_LitigationCaseAxis
= fibk_KellyLoggingInc.v.FirstInterstateBankMember
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember		  
Loss Contingency, Punitive Damages Awarded, Value   16,760fibk_LossContingencyPunitiveDamagesAwardedValue
/ us-gaap_LitigationCaseAxis
= fibk_KellyLoggingInc.v.FirstInterstateBankMember
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember		  
Loss Contingency Accrual     $ 4,000us-gaap_LossContingencyAccrualAtCarryingValue
/ us-gaap_LitigationCaseAxis
= fibk_KellyLoggingInc.v.FirstInterstateBankMember
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember