Subordinated Debentures (Details 1)	Dec. 31, 2012
Subordinated Debentures (Textual) [Abstract]
Size of company eligible for Tier 1 grandfather provision	15,000,000,000

Size of company eligible for Tier 1 grandfather provision.

No definition available.

Name:	gbci_SizeOfCompanyEligibleForTier1GrandfatherProvision
Namespace Prefix:	gbci_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant