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Stock-Based Compensation - Narrative (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award      
Stock compensation expense $ 3,014us-gaap_AllocatedShareBasedCompensationExpense $ 2,936us-gaap_AllocatedShareBasedCompensationExpense $ 2,485us-gaap_AllocatedShareBasedCompensationExpense
Tax benefits from stock-based compensation 1,153us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense 1,122us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense 950us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award      
Stock compensation expense 905us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 1,390us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 1,276us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
Tax benefits from stock-based compensation 346us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 531us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 488us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
Intrinsic value of fully-vested stock options outstanding 14,879us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		    
Total intrinsic value of options exercised 7,363us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 7,108us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 1,158us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
Tax benefit realized from stock option exercises 2,594us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 1,963us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 397us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
Proceeds from stock options exercised 6,299us-gaap_ProceedsFromStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 9,271us-gaap_ProceedsFromStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 1,612us-gaap_ProceedsFromStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
Common stock redeemed for payment of stock option exercises, aggregate value 6,829fibk_CommonStockRedeemedForPaymentOfStockOptionExercisesAggregateValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 8,721fibk_CommonStockRedeemedForPaymentOfStockOptionExercisesAggregateValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		 2,675fibk_CommonStockRedeemedForPaymentOfStockOptionExercisesAggregateValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 577us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		    
Unrecognized compensation cost related to nonvested restricted stock to be recognized over weighted average period (in years) 0 years 10 months 9 days    
Share-Based CompensationaArrangement By Share Based Payment Award, Options, Vested In Period, Fair Value 1,363fibk_ShareBasedCompensationaarrangementByShareBasedPaymentAwardOptionsVestedInPeriodFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember		    
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award      
Stock compensation expense 2,109us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember		 1,546us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember		 1,209us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Tax benefits from stock-based compensation 807us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember		 591us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember		 462us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Unrecognized compensation cost related to nonvested restricted stock to be recognized over weighted average period (in years) 1 year 7 months 6 days    
Stock issued during period, shares, restricted stock award 148,278us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember		    
Unrecognized compensation cost related to nonvested restricted stock awards $ 3,353us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember		    
Restricted Stock | Upon Achievement of Performance Goals      
Share-based Compensation Arrangement by Share-based Payment Award      
Stock issued during period, shares, restricted stock award 73,938us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_VestingAxis
= fibk_UponAchievementOfPerformanceGoalsMember		    
Restricted Stock | Upon Achievement of Defined Return on Asset Performance Goals      
Share-based Compensation Arrangement by Share-based Payment Award      
Stock issued during period, shares, restricted stock award 24,646us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_VestingAxis
= fibk_UponAchievementOfDefinedReturnOnAssetPerformanceGoalsMember		    
Restricted Stock | Upon Achievement of Defined Return on Equity Performance Goals      
Share-based Compensation Arrangement by Share-based Payment Award      
Stock issued during period, shares, restricted stock award 24,646us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_VestingAxis
= fibk_UponAchievementOfDefinedReturnOnEquityPerformanceGoalsMember		    
Restricted Stock | Upon Achievement of Defined Total Return to Shareholder Goals      
Share-based Compensation Arrangement by Share-based Payment Award      
Stock issued during period, shares, restricted stock award 24,646us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_VestingAxis
= fibk_UponAchievementofDefinedTotalReturntoShareholderGoalsDomain		    
Restricted Stock | Upon One-Third Annual Anniversary      
Share-based Compensation Arrangement by Share-based Payment Award      
Stock issued during period, shares, restricted stock award 74,340us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_VestingAxis
= fibk_UponOneThirdAnnualAnniversaryMember		    
2006 Plan | Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award      
Shares available for grant 783,007us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
/ us-gaap_PlanNameAxis
= fibk_TwoThousandSixPlanMember		    
2006 Plan | Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award      
Restricted stock award, vesting period 3 years