Investment Securities, Available-for-Sale (Details Textual) (Residential mortgage-backed securities [Member], USD $)	Sep. 30, 2012	Dec. 31, 2011
Residential mortgage-backed securities [Member]
Investment Securities, Available-for-Sale (Textual) [Abstract]
Non-guaranteed private label whole loan mortgage-backed securities	$ 49,441,000	$ 49,252,000

Non-guaranteed private label whole loan mortgage backed securities available for sale disclosure.

No definition available.

Name:	gbci_NonGuaranteedPrivateLabelWholeLoanMortgageBackedSecuritiesAvailableForSaleDisclosure
Namespace Prefix:	gbci_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant