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Acquisitions Finite Lived Intangible Assets Acquired as Part of a Business Combination (Details) (Core Deposits Intangibles, USD $)
In Thousands, unless otherwise specified
 0 Months Ended 3 Months Ended
Jul. 31, 2014
Sep. 30, 2014
Acquired Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 10 years  
Mountain West Bank    
Acquired Finite-Lived Intangible Assets [Line Items]    
Finite-lived Intangible Assets Acquired   $ 11,014us-gaap_FinitelivedIntangibleAssetsAcquired1
/ us-gaap_BusinessAcquisitionAxis
= fibk_MountainWestBankMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember