Loans Receivable, Net - Troubled Debt Restructurings (Details) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2022 USD ($) Loan	Jun. 30, 2021 USD ($) Loan	Jun. 30, 2022 USD ($) Loan	Jun. 30, 2021 USD ($) Loan
TDRs that occurred during the period
Number of loans \| Loan	2	3	5	10
Pre-modification recorded balance	$ 1,932	$ 615	$ 2,019	$ 2,368
Post-modification recorded balance	$ 1,932	$ 615	$ 2,019	$ 2,368
TDRs that subsequently defaulted
Number of loans \| Loan	0	0	0	0
Recorded balance	$ 0	$ 0	$ 0	$ 0
Residential real estate
TDRs that occurred during the period
Number of loans \| Loan	0	0	1	1
Pre-modification recorded balance	$ 0	$ 0	$ 31	$ 210
Post-modification recorded balance	$ 0	$ 0	$ 31	$ 210
TDRs that subsequently defaulted
Number of loans \| Loan	0	0	0	0
Recorded balance	$ 0	$ 0	$ 0	$ 0
Commercial real estate
TDRs that occurred during the period
Number of loans \| Loan	2	1	2	5
Pre-modification recorded balance	$ 1,932	$ 99	$ 1,932	$ 1,473
Post-modification recorded balance	$ 1,932	$ 99	$ 1,932	$ 1,473
TDRs that subsequently defaulted
Number of loans \| Loan	0	0	0	0
Recorded balance	$ 0	$ 0	$ 0	$ 0
Other commercial
TDRs that occurred during the period
Number of loans \| Loan	0	2	2	3
Pre-modification recorded balance	$ 0	$ 516	$ 56	$ 554
Post-modification recorded balance	$ 0	$ 516	$ 56	$ 554
TDRs that subsequently defaulted
Number of loans \| Loan	0	0	0	0
Recorded balance	$ 0	$ 0	$ 0	$ 0
Home equity
TDRs that occurred during the period
Number of loans \| Loan	0	0	0	0
Pre-modification recorded balance	$ 0	$ 0	$ 0	$ 0
Post-modification recorded balance	$ 0	$ 0	$ 0	$ 0
TDRs that subsequently defaulted
Number of loans \| Loan	0	0	0	0
Recorded balance	$ 0	$ 0	$ 0	$ 0
Other consumer
TDRs that occurred during the period
Number of loans \| Loan	0	0	0	1
Pre-modification recorded balance	$ 0	$ 0	$ 0	$ 131
Post-modification recorded balance	$ 0	$ 0	$ 0	$ 131
TDRs that subsequently defaulted
Number of loans \| Loan	0	0	0	0
Recorded balance	$ 0	$ 0	$ 0	$ 0

Number of financing receivables that have been modified by troubled debt restructurings.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI https://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI https://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI https://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gbci_HomeEquityPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: