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Subordinated Debentures (Details Textual) | Dec. 31, 2013
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Debt Instruments [Abstract] |
Size of company eligible for Tier 1 grandfather provision | 15,000,000,000

X
- Definition Size of company eligible for Tier 1 grandfather provision. + References No definition available. + Details
Name: | gbci_SizeOfCompanyEligibleForTier1GrandfatherProvision
Namespace Prefix: | gbci_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

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