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Stock-based Compensation Plan (Details Textual) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Stock-based Compensation Plans      
Shares available to grant to employees and directors 4,022,452us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant    
Stock Options      
Stock Options      
Expiration date of stock options granted 5 years    
Stock options granted during year 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Unrecognized compensation cost, net of tax $ 0us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
   
Total intrinsic value of options exercised 778,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
1,907,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
3,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Total intrinsic value of options exercised, tax benefit 302,000us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
742,000us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
1,000us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Cash received from options exercised 871,000us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
4,327,000us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
81,000us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Average remaining life on stock options outstanding 6 months    
Average remaining life on stock options exercisable 6 months    
Aggregate intrinsic value of stock options outstanding 11,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
   
Aggregate intrinsic value of stock options exercisable 11,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
   
Stock Options | Director      
Stock Options      
Requisite service period of stock options granted 6 months    
Stock Options | Employee      
Stock Options      
Requisite service period of stock options granted 2 years    
Restricted Stock      
Restricted Stock Awards      
Compensation cost for stock compensation plans 1,603,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
768,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
243,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Income tax benefit from stock compensation plans 622,000us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
299,000us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
96,000us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Unrecognized compensation cost, net of tax 2,276,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Unrecognized compensation costs, recognized over weighted-average period 1 year 11 months    
Fair value of shares vested 953,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
197,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
243,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Tax benefit of shares vested $ 532,000gbci_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTaxBenefit
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
$ 77,000gbci_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTaxBenefit
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
$ 96,000gbci_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTaxBenefit
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember