Name:	gbci_QualifiedAffordableHousingProjectInvestmentsCommitmentToBePaidYearOne
Namespace Prefix:	gbci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of commitment or contingent commitments to to be paid to qualified affordable housing project investments in year two.

No definition available.

Name:	gbci_QualifiedAffordableHousingProjectInvestmentsCommitmentToBePaidYearTwo
Namespace Prefix:	gbci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The period in which a tax credit project must stay in compliance for the investor to keep the tax credits.

No definition available.

Name:	gbci_TaxCreditCompliancePeriod
Namespace Prefix:	gbci_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The period in which an investor receives tax credits from a tax credit project.

No definition available.

Name:	gbci_TaxCreditPeriod
Namespace Prefix:	gbci_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Amount of write-down from impairment of an investment in a qualified affordable housing project.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=64813897&loc=SL49122993-202985

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

Amount of investment in qualified affordable housing projects.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64813897&loc=SL49122993-202985

Name:	us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of commitment or contingent commitments to qualified affordable housing project investments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=64813897&loc=SL49122993-202985

Name:	us-gaap_QualifiedAffordableHousingProjectInvestmentsCommitment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.