Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Details Textual)	Dec. 31, 2012	Apr. 30, 2012	Apr. 30, 2012 Segment Subsidiary
Fair Value of Assets and Liabilities (Textual) [Abstract]
Number of Bank Subsidiaries Prior to Combination of Bank Subsidiaries		11	11
Number of Bank Division Reporting Units Aggregated for Goodwill Assessment	11	11	11
Number of Operating Segments Subsequent to Charter Consolidation		1	1

Number of operating segments subsequent to combination of bank subsidiaries

No definition available.

Name:	gbci_NumberOfOperatingSegmentsSubsequentToCombinationOfBankSubsidiaries
Namespace Prefix:	gbci_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant