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Cash on Hand and in Banks (Details Textual) (USD $) | Dec. 31, 2014
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Cash and Cash Equivalents [Abstract] |
Federal Reserve balance or cash on hand required | $ 33,177,000gbci_FederalReserveBalanceOrCashOnHandRequired

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- Definition Federal Reserve balance or cash on hand required. + References No definition available. + Details
Name: | gbci_FederalReserveBalanceOrCashOnHandRequired
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_CashAndCashEquivalentsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration