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Contingencies and Commitments (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Outstanding Commitments    
Outstanding commitments $ 976,711us-gaap_OtherCommitment $ 881,550us-gaap_OtherCommitment
Commitments to Extend Credit    
Outstanding Commitments    
Outstanding commitments 960,180us-gaap_OtherCommitment
/ us-gaap_OtherCommitmentsAxis
= gbci_CommitmenttoExtendCreditMember		 866,885us-gaap_OtherCommitment
/ us-gaap_OtherCommitmentsAxis
= gbci_CommitmenttoExtendCreditMember
Letters of Credit    
Outstanding Commitments    
Outstanding commitments $ 16,531us-gaap_OtherCommitment
/ us-gaap_OtherCommitmentsAxis
= gbci_LettersofCreditMember		 $ 14,665us-gaap_OtherCommitment
/ us-gaap_OtherCommitmentsAxis
= gbci_LettersofCreditMember