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

Capital Conservation Buffer as a percentage of risk-weighted assets required for the current year.

No definition available.

Name:	gbci_CapitalConservationBufferToRiskWeightedAssetsCurrentYear
Namespace Prefix:	gbci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant