Leases - Summary (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Finance Leases
ROU assets	$ 5,999	$ 5,999
Accumulated depreciation	$ (396)	$ (273)
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	gbci:PropertyPlantAndEquipmentAndLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization	gbci:PropertyPlantAndEquipmentAndLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization
Net ROU assets	$ 5,603	$ 5,726
Finance Lease, Liability, Statement of Financial Position [Extensible Enumeration]	us-gaap:OtherBorrowings	us-gaap:OtherBorrowings
Lease liabilities	$ 5,837	$ 5,891
Operating Leases
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	gbci:PropertyPlantAndEquipmentAndLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization	gbci:PropertyPlantAndEquipmentAndLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization
Net ROU assets	$ 45,273	$ 46,820
Operating Lease, Liability, Statement of Financial Position [Extensible Enumeration]	us-gaap:OtherLiabilities	us-gaap:OtherLiabilities
Lease liabilities	$ 48,296	$ 49,675
Finance lease, weighted average remaining lease term	23 years	24 years
Finance lease, weighted average discount rate	2.60%	2.60%
Operating lease, weighted average remaining lease term	17 years	17 years
Operating lease, weighted average discount rate	3.40%	3.40%