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Contingencies and Commitments (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Outstanding Commitments |  |
Outstanding commitments | $ 1,605,194 | $ 1,351,398
Unused Lines of Credit |  |
Outstanding Commitments |  |
Outstanding commitments | 1,565,112 | 1,325,236
Letters of Credit |  |
Outstanding Commitments |  |
Outstanding commitments | $ 40,082 | $ 26,162

X
- Definition Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions. + References No definition available. + Details
Name: | us-gaap_OtherCommitment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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

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- Details
Name: | us-gaap_OtherCommitmentsAxis=gbci_CommitmenttoExtendCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_OtherCommitmentsAxis=gbci_LettersofCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |