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Loans Receivable, Net (Details Textual) | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2018 USD ($) portfolio_segment | Dec. 31, 2017 USD ($) | Mar. 31, 2017 USD ($)
Loans and Leases Receivable Disclosure |  |  |
Number of segments in the Company's loan portfolio | portfolio_segment | 3 |  |
TDR With Pre Modification Loan Balance for Which Oreo Was Received | $ 431,000 |  | $ 514,000
Consumer mortgage loans secured by residential real estate properties for which formal foreclosure proceedings are in process | 1,885,000 | $ 743,000 |
OREO Secured by Residential Real Estate | $ 1,025,000 | $ 893,000 |

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- Definition Number of portfolio segments in the company's total financing receivables. + References No definition available. + Details
Name: | gbci_FinancingReceivableNumberofPortfolioSegments
Namespace Prefix: | gbci_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Real Estate Acquired Through Foreclosure, Secured by Residential Real Estate + References No definition available. + Details
Name: | gbci_RealEstateAcquiredThroughForeclosureSecuredbyResidentialRealEstate
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition TDR with pre modification loan balance, for which OREO was received. + References No definition available. + Details
Name: | gbci_TdrWithPreModificationLoanBalanceForWhichOreoWasReceived
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_LoansAndLeasesReceivableDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 35 -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL49126937-111524 + Details
Name: | us-gaap_MortgageLoansInProcessOfForeclosureAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant