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Interest Rate Swap Summary (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Interest Rate Swap One |  |
Interest rate swap derivative financial instruments |  |
Forecasted Notional Amount | $ 160,000us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_DerivativeByNatureAxis = gbci_InterestRateSwapOneMember |
Variable Interest Rate | 3 month LIBOR | [1]
Fixed Interest Rate | 3.378%us-gaap_DerivativeFixedInterestRate / us-gaap_DerivativeByNatureAxis = gbci_InterestRateSwapOneMember | [1]
Term, Effective Date | Oct. 21, 2014 | [2]
Term, Maturity Date | Oct. 21, 2021 | [2]
Interest Rate Swap Two |  |
Interest rate swap derivative financial instruments |  |
Forecasted Notional Amount | $ 100,000us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_DerivativeByNatureAxis = gbci_InterestRateSwapTwoMember |
Variable Interest Rate | 3 month LIBOR | [1]
Fixed Interest Rate | 2.498%us-gaap_DerivativeFixedInterestRate / us-gaap_DerivativeByNatureAxis = gbci_InterestRateSwapTwoMember | [1]
Term, Effective Date | Nov. 30, 2015 | [2]
Term, Maturity Date | Nov. 30, 2022 | [2]
[1] | The Company pays the fixed interest rate and the counterparty pays the Company the variable interest rate.
[2] | No cash will be exchanged prior to the beginning of the payment term.

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- Definition The reference rate for the variable rate of the interest rate derivative, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR. + References No definition available. + Details
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Namespace Prefix: | gbci_
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Balance Type: | na
Period Type: | duration

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- Definition Fixed interest rate related to the interest rate derivative. + References No definition available. + Details
Name: | us-gaap_DerivativeFixedInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Date the entity entered into the derivative contract, in CCYY-MM-DD format. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition Nominal or face amount used to calculate payments on the derivative liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Notional Amount -URI http://asc.fasb.org/extlink&oid=6519104 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | instant

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- Details
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Period Type: | duration

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- Definition Date the derivative contract matures, in CCYY-MM-DD format. + References No definition available. + Details
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |