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Pro Forma Summary (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
2014 acquisitions      
Pro Forma Information      
Net interest income and non-interest income $ 371,772us-gaap_BusinessAcquisitionsProFormaRevenue
/ us-gaap_BusinessAcquisitionAxis
= gbci_CurrentYearAcquisitionsMember		 $ 340,393us-gaap_BusinessAcquisitionsProFormaRevenue
/ us-gaap_BusinessAcquisitionAxis
= gbci_CurrentYearAcquisitionsMember		  
Net income 113,364us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= gbci_CurrentYearAcquisitionsMember		 99,275us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= gbci_CurrentYearAcquisitionsMember		  
2013 acquisitions      
Pro Forma Information      
Net interest income and non-interest income   339,236us-gaap_BusinessAcquisitionsProFormaRevenue
/ us-gaap_BusinessAcquisitionAxis
= gbci_PriorYearAcquisitionsMember		 334,317us-gaap_BusinessAcquisitionsProFormaRevenue
/ us-gaap_BusinessAcquisitionAxis
= gbci_PriorYearAcquisitionsMember
Net income   $ 96,392us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= gbci_PriorYearAcquisitionsMember		 $ 80,403us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= gbci_PriorYearAcquisitionsMember