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Capital Stock and Dividend Restrictions (Details) (USD $)
 12 Months Ended 0 Months Ended 9 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jul. 31, 2014
Sep. 30, 2014
Jan. 24, 2014
Preferred Stock            
Nonvoting, noncumulative preferred stock, shares outstanding 0us-gaap_PreferredStockSharesOutstanding 0us-gaap_PreferredStockSharesOutstanding        
Common Stock, Recapitalization            
Common stock, shares authorized 200,000,000us-gaap_CommonStockSharesAuthorized          
Common Stock            
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned $ 35,972,000us-gaap_BusinessAcquisitionEquityInterestIssuedOrIssuableValueAssigned          
Payments for Repurchase of Common Stock 9,739,000us-gaap_PaymentsForRepurchaseOfCommonStock 448,000us-gaap_PaymentsForRepurchaseOfCommonStock 263,000us-gaap_PaymentsForRepurchaseOfCommonStock      
Series A Preferred Stock            
Preferred Stock            
Nonvoting, noncumulative preferred stock, shares outstanding 5,000,000us-gaap_PreferredStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember		          
Preferred stock, value, issued 50,000,000us-gaap_PreferredStockValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember		          
Preferred stock, dividend rate, per-dollar-amount $ 675us-gaap_PreferredStockDividendRatePerDollarAmount
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember		          
Preferred stock, aggregate redemption price 50,150fibk_PreferredStockAggregateRedemptionPrice
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember		          
Preferred stock, redemption price per share $ 10,000us-gaap_PreferredStockRedemptionPricePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember		          
Class B Common Stock            
Common Stock, Recapitalization            
Common stock, shares authorized 100,000,000us-gaap_CommonStockSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember		          
Common Stock, Voting Rights 5          
Common Stock            
Common stock, shares, outstanding 23,859,483us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember		 24,287,045us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember		        
Class A Common Stock            
Class of Stock            
Stock Repurchased and Retired During Period, Weighted Average Price $ 25.07fibk_StockRepurchasedandRetiredDuringPeriodWeightedAveragePrice
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		          
Common Stock, Recapitalization            
Common stock, shares authorized 100,000,000us-gaap_CommonStockSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		          
Common Stock, Voting Rights 1          
Common Stock            
Common stock, shares, outstanding 21,928,932us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		 19,868,018us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		        
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned       35,972,000us-gaap_BusinessAcquisitionEquityInterestIssuedOrIssuableValueAssigned
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		    
Stock Repurchased and Retired During Period, Shares 362,121us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		          
Payments for Repurchase of Common Stock 9,078,000us-gaap_PaymentsForRepurchaseOfCommonStock
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		          
Amendment to 2006 Equity Compensation Plan [Domain] | Class A Common Stock            
Class of Stock            
Additional Shares Authorized           1,500,000fibk_AdditionalSharesAuthorized
/ us-gaap_PlanNameAxis
= fibk_Amendmentto2006EquityCompensationPlanDomain
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
Mountain West Bank | Class A Common Stock            
Common Stock            
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares 1,378,230us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
/ us-gaap_BusinessAcquisitionAxis
= fibk_MountainWestBankMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		     1,378,230us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
/ us-gaap_BusinessAcquisitionAxis
= fibk_MountainWestBankMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		    
Board of Directors [Member]            
Common Stock            
Share-based Compensation Arrangement by Share-based Payment Award, Aggregate Value of Shares Issued   $ 543,000fibk_SharebasedCompensationArrangementbySharebasedPaymentAwardAggregateValueofSharesIssued
/ us-gaap_TitleOfIndividualAxis
= fibk_BoardofDirectorsMember		     $ 620,000fibk_SharebasedCompensationArrangementbySharebasedPaymentAwardAggregateValueofSharesIssued
/ us-gaap_TitleOfIndividualAxis
= fibk_BoardofDirectorsMember		  
Board of Directors [Member] | Class A Common Stock            
Common Stock            
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period   26,096us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
/ us-gaap_TitleOfIndividualAxis
= fibk_BoardofDirectorsMember		     24,581us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
/ us-gaap_TitleOfIndividualAxis
= fibk_BoardofDirectorsMember