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Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Summary of investment securities |  |
Amortized Cost | $ 2,862,764gbci_DebtSecuritiesAmortizedCostBasis | $ 3,208,941gbci_DebtSecuritiesAmortizedCostBasis
Gross Unrealized Gains | 86,393gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments | 49,801gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments
Gross Unrealized Losses | (10,783)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments | (35,913)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments
Fair Value | 2,938,374gbci_DebtSecuritiesFairValue | 3,222,829gbci_DebtSecuritiesFairValue
Available-for-sale securities |  |
Summary of investment securities |  |
Amortized Cost | 2,341,767gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember | 3,208,941gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember
Gross Unrealized Gains | 53,468gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember | 49,801gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember
Gross Unrealized Losses | (7,807)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember | (35,913)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember
Fair Value | 2,387,428gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember | 3,222,829gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember
Available-for-sale securities | U.S. government and federal agency |  |
Summary of investment securities |  |
Amortized Cost | 44gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Gross Unrealized Gains | 0gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Gross Unrealized Losses | 0gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Fair Value | 44gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Available-for-sale securities | U.S. government sponsored enterprises |  |
Summary of investment securities |  |
Amortized Cost | 21,916gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 10,441gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Gross Unrealized Gains | 31gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 187gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Gross Unrealized Losses | (2)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 0gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Fair Value | 21,945gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 10,628gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Available-for-sale securities | State and local governments |  |
Summary of investment securities |  |
Amortized Cost | 962,365gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 1,377,347gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross Unrealized Gains | 40,173gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 31,621gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross Unrealized Losses | (4,569)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | (23,890)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Fair Value | 997,969gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 1,385,078gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Available-for-sale securities | Corporate bonds |  |
Summary of investment securities |  |
Amortized Cost | 313,545gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 440,337gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Gross Unrealized Gains | 2,059gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 3,922gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Gross Unrealized Losses | (750)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | (1,758)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Fair Value | 314,854gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 442,501gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Available-for-sale securities | Residential mortgage-backed securities |  |
Summary of investment securities |  |
Amortized Cost | 1,043,897gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 1,380,816gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Gross Unrealized Gains | 11,205gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 14,071gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Gross Unrealized Losses | (2,486)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | (10,265)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Fair Value | 1,052,616gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 1,384,622gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Held-to-maturity securities |  |
Summary of investment securities |  |
Amortized Cost | 520,997gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember |
Gross Unrealized Gains | 32,925gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember |
Gross Unrealized Losses | (2,976)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember |
Fair Value | 550,946gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember |
Held-to-maturity securities | State and local governments |  |
Summary of investment securities |  |
Amortized Cost | 520,997gbci_DebtSecuritiesAmortizedCostBasis / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Gross Unrealized Gains | 32,925gbci_SecuritiesGrossUnrealizedGainAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Gross Unrealized Losses | (2,976)gbci_SecuritiesGrossUnrealizedLossAccumulatedinInvestments / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Fair Value | $ 550,946gbci_DebtSecuritiesFairValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |

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- Definition This item represents the cost of available-for-sale and held-to-maturity debt securities, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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- Definition Amount of accumulated unrealized gain before deducting unrealized loss on investments in available-for-sale and held-to-maturity securities impacting investments. + References No definition available. + Details
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