Investment Securities, Available-for-Sale (Details Textual) (Residential mortgage-backed securities, USD $)	Sep. 30, 2013	Dec. 31, 2012
Residential mortgage-backed securities
Investment Securities, Available-for-Sale (Textual) [Abstract]
Non-guaranteed private label whole loan mortgage-backed securities	$ 3,214,000	$ 46,733,000

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

No definition available.

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