Premises and Equipment (Details Textual) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Premises and Equipment
Depreciation expense	$ 12,108,000us-gaap_DepreciationNonproduction	$ 10,485,000us-gaap_DepreciationNonproduction	$ 10,615,000us-gaap_DepreciationNonproduction
Total rent expense	2,786,000us-gaap_LeaseAndRentalExpense	2,912,000us-gaap_LeaseAndRentalExpense	2,868,000us-gaap_LeaseAndRentalExpense
Related party rent expense	$ 146,000gbci_RelatedPartyRentExpense	$ 142,000gbci_RelatedPartyRentExpense	$ 410,000gbci_RelatedPartyRentExpense

The expense recognized in the current period that allocates the cost of nonproduction tangible assets over their useful lives.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Name:	us-gaap_DepreciationNonproduction
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration