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Variable Interest Entities VIE Net Income or Loss and Tax Credits (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
CDE (NMTC)    
Consolidated VIEs    
VIE income (loss) $ 209gbci_VariableInterestEntityConsolidatedIncomeorLossNet
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= gbci_CertifiedDevelopmentEntitiesMember		 $ 209gbci_VariableInterestEntityConsolidatedIncomeorLossNet
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= gbci_CertifiedDevelopmentEntitiesMember
Federal income tax credits 0gbci_VariableInterestEntityConsolidatedTaxCreditAmount
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= gbci_CertifiedDevelopmentEntitiesMember		 0gbci_VariableInterestEntityConsolidatedTaxCreditAmount
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= gbci_CertifiedDevelopmentEntitiesMember
LIHTC    
Consolidated VIEs    
VIE income (loss) (252)gbci_VariableInterestEntityConsolidatedIncomeorLossNet
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= gbci_LowIncomeHousingTaxCreditMember		 (270)gbci_VariableInterestEntityConsolidatedIncomeorLossNet
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= gbci_LowIncomeHousingTaxCreditMember
Federal income tax credits $ 294gbci_VariableInterestEntityConsolidatedTaxCreditAmount
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= gbci_LowIncomeHousingTaxCreditMember		 $ 318gbci_VariableInterestEntityConsolidatedTaxCreditAmount
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= gbci_LowIncomeHousingTaxCreditMember