Future Unfunded Contingent Commitments (Details) - Qualified Affordable Housing Project Investments $ in Thousands	Jun. 30, 2019 USD ($)
Other Commitments
Unfunded contingent commitments expected to be fulfilled in 2019	$ 7,004
Unfunded contingent commitments expected to be fulfilled in 2020	3,042
Unfunded contingent commitments expected to be fulfilled in 2021	2,640
Unfunded contingent commitments expected to be fulfilled in 2022	7,101
Unfunded contingent commitments expected to be fulfilled in 2023	59
Unfunded contingent commitments expected to be fulfilled thereafter	641
Unfunded contingent commitments	$ 20,487

Qualified Affordable Housing Project Investments, Commitment To Be Paid Thereafter

No definition available.

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

- Definition

Qualified Affordable Housing Project Investments, Commitment To Be Paid Year Five

+ References

No definition available.

+ Details

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

- Definition

Qualified Affordable Housing Project Investments, Commitment To Be Paid Year Four

+ References

No definition available.

+ Details

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

- Definition

Qualified Affordable Housing Project Investments, Commitment To Be Paid Year One

+ References

No definition available.

+ Details

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

- Definition

Qualified Affordable Housing Project Investments, Commitment To Be Paid Year Three

+ References

No definition available.

+ Details

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

- Definition

Qualified Affordable Housing Project Investments, Commitment To Be Paid Year Two

+ References

No definition available.

+ Details

Name:	gbci_QualifiedAffordableHousingProjectInvestmentsCommitmentToBePaidYearTwo
Namespace Prefix:	gbci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_OtherCommitmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ 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 (e)
-URI http://asc.fasb.org/extlink&oid=114775393&loc=SL49122993-202985

+ Details

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

- Details

Name:	us-gaap_OtherCommitmentsAxis=gbci_QualifiedAffordableHousingProjectInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: