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Subordinated Debentures (Details Textual)
12 Months Ended
Dec. 31, 2014
Debt Instruments [Abstract]  
Maximum Number of Consecutive Quarters for Deferral of Interest Payments on Trust Preferred Securities 20
Size of company eligible for Tier 1 grandfather provision 15,000,000,000gbci_SizeOfCompanyEligibleForTier1GrandfatherProvision