Fair Value of Financial Instruments (Details 1) (USD $) In Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Reconciliation of beginning and ending balances for assets measured on a recurring basis
Beginning Balance	$ 6,751	$ 9,988
Total unrealized gains included in other comprehensive income	1,641	3,381
Amortization, accretion and principal payments	(2,240)	(1,510)
Sales, maturities and calls		(3,020)
Transfers out of Level 3		(2,088)
Ending Balance	6,152	6,751
State and local governments and other issues [Member]
Reconciliation of beginning and ending balances for assets measured on a recurring basis
Beginning Balance		2,088
Total unrealized gains included in other comprehensive income		0
Amortization, accretion and principal payments		0
Sales, maturities and calls		0
Transfers out of Level 3		(2,088)
Ending Balance	0	0
Collateralized debt obligations [Member]
Reconciliation of beginning and ending balances for assets measured on a recurring basis
Beginning Balance	6,595	6,789
Total unrealized gains included in other comprehensive income	1,598	3,276
Amortization, accretion and principal payments	(2,240)	(1,510)
Sales, maturities and calls		(1,960)
Transfers out of Level 3		0
Ending Balance	5,953	6,595
Residential Mortgage Backed Securities [Member]
Reconciliation of beginning and ending balances for assets measured on a recurring basis
Beginning Balance	156	1,111
Total unrealized gains included in other comprehensive income	43	105
Amortization, accretion and principal payments	0	0
Sales, maturities and calls		(1,060)
Transfers out of Level 3		0
Ending Balance	$ 199	$ 156

Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Amortization Accretion And Principal Payments.

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Namespace Prefix:	gbci
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents total gains or losses for the period (realized and unrealized), arising from assets measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in other comprehensive income (a separate component of shareholders' equity).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 157
-Paragraph 32
-Subparagraph c(1)
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

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

- Definition

This element represents purchases, sales, issuances, and settlements (net) which have taken place during the period in relation to assets measured at fair value on a recurring basis using unobservable inputs (Level 3).

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlements
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents [net] transfers in to and out of assets measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 157
-Paragraph 32
-Subparagraph c(3)
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

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

- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 157
-Paragraph 32
-Subparagraph c
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

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