Loans Receivable, Net (Details Textual) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Loans Receivable
Percentage of consolidated stockholders equity exceeding which no borrower had outstanding loans or commitments	10.00%
Net deferred fees, premiums, and discounts included in the loan receivable	$ 13,710,000	$ 10,662,000
Variable rate loans	2,915,617,000
Fixed rate loans	1,572,478,000
Weighted average interest rate on loans	4.86%gbci_WeightedAverageInterestRateOnLoans	5.04%gbci_WeightedAverageInterestRateOnLoans
Loan sold and serviced for others	133,768,000us-gaap_ServicingAssetAtAmortizedValue	148,376,000us-gaap_ServicingAssetAtAmortizedValue	116,439,000us-gaap_ServicingAssetAtAmortizedValue
Loans pledged as collateral	2,596,010,000us-gaap_LoansPledgedAsCollateral
Aggregate amount of loans outstanding to related parties	49,446,000	35,224,000
New loans to related parties	24,380,000
Repayments of loans by related parties	9,864,000
Loans and Leases Receivable, Related Parties, Change in Composition	(294,000)
Interest income that would have been recorded on non-accrual loans	3,005,000	4,122,000	5,161,000
TDR With Pre Modification Loan Balance for Which Oreo Was Received	12,674,000	18,345,000	39,769,000
Foreclosure in Process, Consumer Mortgage Loans Secured by Residential Real Estate	698,000
OREO Secured by Residential Real Estate	2,322,000
Additional outstanding commitments on TDRs	4,263,000	2,024,000
Charge-offs on TDRs	$ 1,361,000	$ 1,945,000	$ 6,271,000

Additional outstanding commitments on TDRs.

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Amount of Consumer Mortgage Loans Secured by Residential Real Estate for which Formal Foreclosure Proceedings are in Process.

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No definition available.

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Reflects the amount of change in related party loan and lease receivable due to the addition or deletion of a specific individual considered to be a related party.

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No definition available.

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Percentage of consolidated stockholders equity exceeding which no borrower had outstanding loans or commitments.

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No definition available.

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Real Estate Acquired Through Foreclosure, Secured by Residential Real Estate

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No definition available.

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TDR with pre modification loan balance, for which OREO was received.

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No definition available.

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- Definition

Weighted average interest rate on loans.

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No definition available.

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- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

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-Name Accounting Standards Codification

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Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

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For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date.

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Reflects the amount of new advances made to related parties in the form of loan and lease receivables.

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Reflects the amount of collections received from related parties to reduce the carrying amounts of loans and leases receivable due from them.

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Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

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Amount of loan with fixed rate of interest.

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Amount of loan with variable or adjustable rate of interest.

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Amortized amount of contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

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Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for loans and leases covered under loss sharing agreements.

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