Variable Interest Entities (Details Textual)	3 Months Ended
Mar. 31, 2016 USD ($)
Variable Interest Entities
Unfunded contingent commitments	$ 4,536,000
Year in which commitments will be fulfilled	2017
Impairment losses	$ 0
Other assets
Variable Interest Entities
Carrying value of equity investments in LIHTCs	$ 4,579,000
CDE
Variable Interest Entities
Tax credit period	7 years
LIHTC
Variable Interest Entities
Tax credit period	10 years
Tax credit compliance period	15 years

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

No definition available.

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Balance Type:	na
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- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	gbci_
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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://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=49127445&loc=SL49122993-202985

+ Details

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

- Definition

Year in which contingent commitments to qualified affordable housing project investments are expected to be paid, in CCYY format.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in qualified affordable housing projects.

+ References

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=49127445&loc=SL49122993-202985

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

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=49127445&loc=SL49122993-202985

+ Details

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Namespace Prefix:	us-gaap_
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

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- Details

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- Details

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