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v3.20.4

Regulatory Capital - Narrative (Details) | Dec. 31, 2020
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Common Equity Tier 1 (to risk-weighted assets) |
Capital conservation buffer | 2.50%

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- Definition Capital Conservation Buffer as a percentage of risk-weighted assets required for the current year. + References No definition available. + Details
Name: | gbci_CapitalConservationBufferToRiskWeightedAssetsCurrentYear
Namespace Prefix: | gbci_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Tier One Common Capital [Abstract] + References No definition available. + Details
Name: | gbci_TierOneCommonCapitalAbstract
Namespace Prefix: | gbci_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration