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Federal and State Income Taxes (Details Textual) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Summary of Tax Credit Carryforwards    
Federal income tax credit carryforwards, Amount $ 845,000us-gaap_TaxCreditCarryforwardAmount  
Federal Income Tax Credit Carryforwards, Expiration Date Dec. 31, 2034  
Income Tax Uncertainties    
Unrecognized income tax benefit 0us-gaap_UnrecognizedTaxBenefits 0us-gaap_UnrecognizedTaxBenefits
Accrued liabilities for the payment of interest or penalties 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued
Valuation Allowance    
Deferred tax valuation allowance 0us-gaap_DeferredTaxAssetsValuationAllowance 0us-gaap_DeferredTaxAssetsValuationAllowance
Federal and State Income Taxes    
Base year reserve for bad debts 3,600,000gbci_BadDebtReserveForTaxPurposesOfSavingsAndLoanAssociationsOrOtherQualifiedThriftLender  
Federal    
Operating Loss Carryforwards    
Operating Loss Carryforwards, Amount 9,388,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
 
Federal | Earliest Tax Year    
Operating Loss Carryforwards    
Operating Loss Carryforwards, Expiration Date Dec. 31, 2029  
Federal | Latest Tax Year    
Operating Loss Carryforwards    
Operating Loss Carryforwards, Expiration Date Dec. 31, 2031  
Colorado    
Operating Loss Carryforwards    
Operating Loss Carryforwards, Amount $ 17,317,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= gbci_ColoradoMember
 
Colorado | Earliest Tax Year    
Operating Loss Carryforwards    
Operating Loss Carryforwards, Expiration Date Dec. 31, 2029  
Colorado | Latest Tax Year    
Operating Loss Carryforwards    
Operating Loss Carryforwards, Expiration Date Dec. 31, 2031