Loans Receivable, Net (Details Textual) (USD $)	Mar. 31, 2015	Mar. 31, 2014
Loans and Leases Receivable Disclosure
TDR With Pre Modification Loan Balance for Which Oreo Was Received	$ 3,595,000	$ 4,413,000
Foreclosure in Process, Consumer Mortgage Loans Secured by Residential Real Estate	1,135,000
OREO Secured by Residential Real Estate	$ 1,891,000

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

No definition available.

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