Fair Value of Assets and Liabilities (Details Textual)	Apr. 30, 2012 Subsidiary
Fair Value of Assets and Liabilities (Textual) [Abstract]
Number of bank subsidiaries prior to combination of bank subsidiaries	11
Number of Bank Reporting Units Prior to Combination of Bank Subsidiaries	11
Number of bank subsidiaries subsequent to combination of bank subsidiaries	1

Number of bank subsidiaries subsequent to combination of bank subsidiaries.

No definition available.

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