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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=114775393&loc=SL49122993-202985

+ Details

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

- Definition

Amount of investment in qualified affordable housing projects.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=114775393&loc=SL49122993-202985

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=gbci_CertifiedDevelopmentEntitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=gbci_LowIncomeHousingTaxCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Variable Interest Entities (Details Textual) - USD ($)	12 Months Ended
Variable Interest Entities (Details Textual) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entities
Impairment losses	$ 0	$ 0	$ 0
Other assets
Variable Interest Entities
Carrying value of equity investments in LIHTCs	$ 41,521,000	$ 35,112,000
CDE
Variable Interest Entities
Tax credit period	7 years
LIHTC
Variable Interest Entities
Tax credit period	10 years
Tax credit compliance period	15 years