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Impaired Loans (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Summary of the impaired loans by portfolio class of loans |  |
Loans with a Specific Valuation Allowance, Recorded Balance | $ 45,688us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment | $ 61,503us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Loans with a Specific Valuation Allowance, Unpaid Principal Balance | 48,477us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance | 63,406us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance
Specific valuation allowance | 11,597us-gaap_ImpairedFinancingReceivableRelatedAllowance | 11,949us-gaap_ImpairedFinancingReceivableRelatedAllowance
Loans with a Specific Valuation Allowance, Average Balance | 53,339us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment | 59,823us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment
Loans without a Specific Valuation Allowance, Recorded Balance | 115,678us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment | 138,177us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Loans without a Specific Valuation Allowance, Unpaid Principal Balance | 145,038us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance | 169,082us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance
Loans without a Specific Valuation Allowance, Average Balance | 128,645us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment | 139,129us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment
Recorded balance | 161,366us-gaap_ImpairedFinancingReceivableRecordedInvestment | 199,680us-gaap_ImpairedFinancingReceivableRecordedInvestment
Unpaid principal balance | 193,515us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance | 232,488us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Average balance | 181,984us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment | 198,952us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Residential Real Estate |  |
Summary of the impaired loans by portfolio class of loans |  |
Loans with a Specific Valuation Allowance, Recorded Balance | 4,110us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | 7,233us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Loans with a Specific Valuation Allowance, Unpaid Principal Balance | 4,276us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | 7,394us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Specific valuation allowance | 853us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | 990us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Loans with a Specific Valuation Allowance, Average Balance | 5,480us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | 7,237us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Recorded Balance | 15,466us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | 16,837us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Unpaid Principal Balance | 16,683us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | 18,033us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Average Balance | 15,580us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | 18,103us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Recorded balance | 19,576us-gaap_ImpairedFinancingReceivableRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | 24,070us-gaap_ImpairedFinancingReceivableRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Unpaid principal balance | 20,959us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | 25,427us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Average balance | 21,060us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | 25,340us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Commercial Real Estate |  |
Summary of the impaired loans by portfolio class of loans |  |
Loans with a Specific Valuation Allowance, Recorded Balance | 27,155us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 23,917us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember
Loans with a Specific Valuation Allowance, Unpaid Principal Balance | 28,048us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 25,331us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember
Specific valuation allowance | 2,967us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 3,763us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember
Loans with a Specific Valuation Allowance, Average Balance | 24,519us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 26,105us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember
Loans without a Specific Valuation Allowance, Recorded Balance | 78,109us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 95,609us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember
Loans without a Specific Valuation Allowance, Unpaid Principal Balance | 100,266us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 119,017us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember
Loans without a Specific Valuation Allowance, Average Balance | 89,015us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 95,808us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember
Recorded balance | 105,264us-gaap_ImpairedFinancingReceivableRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 119,526us-gaap_ImpairedFinancingReceivableRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember
Unpaid principal balance | 128,314us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 144,348us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember
Average balance | 113,534us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 121,913us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember
Other Commercial |  |
Summary of the impaired loans by portfolio class of loans |  |
Loans with a Specific Valuation Allowance, Recorded Balance | 11,377us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 27,015us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Loans with a Specific Valuation Allowance, Unpaid Principal Balance | 12,461us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 27,238us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Specific valuation allowance | 6,836us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 6,155us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Loans with a Specific Valuation Allowance, Average Balance | 19,874us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 22,460us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Recorded Balance | 13,944us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 14,489us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Unpaid Principal Balance | 19,117us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 19,156us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Average Balance | 14,024us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 14,106us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Recorded balance | 25,321us-gaap_ImpairedFinancingReceivableRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 41,504us-gaap_ImpairedFinancingReceivableRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Unpaid principal balance | 31,578us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 46,394us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Average balance | 33,898us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 36,566us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Home Equity |  |
Summary of the impaired loans by portfolio class of loans |  |
Loans with a Specific Valuation Allowance, Recorded Balance | 1,214us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember | 886us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember
Loans with a Specific Valuation Allowance, Unpaid Principal Balance | 1,336us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember | 949us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember
Specific valuation allowance | 447us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember | 265us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember
Loans with a Specific Valuation Allowance, Average Balance | 1,039us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember | 767us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Recorded Balance | 5,687us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember | 8,153us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Unpaid Principal Balance | 6,403us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember | 9,631us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Average Balance | 7,163us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember | 8,844us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember
Recorded balance | 6,901us-gaap_ImpairedFinancingReceivableRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember | 9,039us-gaap_ImpairedFinancingReceivableRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember
Unpaid principal balance | 7,739us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember | 10,580us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember
Average balance | 8,202us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember | 9,611us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = gbci_HomeEquityPortfolioSegmentMember
Other Consumer |  |
Summary of the impaired loans by portfolio class of loans |  |
Loans with a Specific Valuation Allowance, Recorded Balance | 1,832us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember | 2,452us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Loans with a Specific Valuation Allowance, Unpaid Principal Balance | 2,356us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember | 2,494us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Specific valuation allowance | 494us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember | 776us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Loans with a Specific Valuation Allowance, Average Balance | 2,427us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember | 3,254us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Recorded Balance | 2,472us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember | 3,089us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Unpaid Principal Balance | 2,569us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember | 3,245us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Loans without a Specific Valuation Allowance, Average Balance | 2,863us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember | 2,268us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Recorded balance | 4,304us-gaap_ImpairedFinancingReceivableRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember | 5,541us-gaap_ImpairedFinancingReceivableRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Unpaid principal balance | 4,925us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember | 5,739us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Average balance | $ 5,290us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember | $ 5,522us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember

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- Definition Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524 + Details
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- Definition Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524 + Details
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- Definition Amount of allowance for credit losses related to recorded investment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524 + Details
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- Definition Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524 + Details
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