Investment Securities, Available-for-Sale (Details Textuals) (Residential Mortgage Backed Securities [Member], USD $)	Jun. 30, 2011	Dec. 31, 2010
Residential Mortgage Backed Securities [Member]
Investment Securities, Available-for-Sale (Textuals) [Abstract]
Non-guaranteed private label whole loan mortgage-backed securities	$ 55,611,000	$ 68,051,000

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gbci_NonGuaranteedPrivateLabelWholeLoanMortgageBackedSecuritiesAvailableForSaleDisclosure

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gbci

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xbrli:monetaryItemType

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debit

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