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Gain or Loss on Sale of Investments (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Available-for-sale |  |  |  |  |  |
Gross proceeds | $ 219,849us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds |  | $ 181,971us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds |  | $ 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds |
Less amortized cost | (220,053)gbci_AvailableforSaleSecuritiesGrossRealizedGainsLossesAmortizedCost | [1] | (182,270)gbci_AvailableforSaleSecuritiesGrossRealizedGainsLossesAmortizedCost | [1] | 0gbci_AvailableforSaleSecuritiesGrossRealizedGainsLossesAmortizedCost | [1]
Net available-for-sale | (204)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet |  | (299)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet |  | 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet |
Gross gain on sale of investments | 501us-gaap_AvailableForSaleSecuritiesGrossRealizedGains |  | 3,723us-gaap_AvailableForSaleSecuritiesGrossRealizedGains |  | 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains |
Gross loss on sale of investments | (705)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses |  | (4,022)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses |  | 0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses |
Held-to-maturity |  |  |  |  |  |
Gross proceeds | 8,930gbci_HeldtoMaturitySecuritiesGrossRealizedGainsLossesSaleProceeds | [2] | 0gbci_HeldtoMaturitySecuritiesGrossRealizedGainsLossesSaleProceeds | [2] | 0gbci_HeldtoMaturitySecuritiesGrossRealizedGainsLossesSaleProceeds | [2]
Less amortized cost | (8,914)gbci_HeldtomaturitySecuritiesGrossRealizedGainsLossesAmortizedCost | [1],[2] | 0gbci_HeldtomaturitySecuritiesGrossRealizedGainsLossesAmortizedCost | [1],[2] | 0gbci_HeldtomaturitySecuritiesGrossRealizedGainsLossesAmortizedCost | [1],[2]
Net held-to-maturity | 16us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss | [2] | 0us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss | [2] | 0us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss | [2]
Gross gain on sale of investments | 22gbci_HeldtoMaturitySecuritiesSoldSecurityRealizedGain | [2] | 0gbci_HeldtoMaturitySecuritiesSoldSecurityRealizedGain | [2] | 0gbci_HeldtoMaturitySecuritiesSoldSecurityRealizedGain | [2]
Gross loss on sale of investments | $ (6)gbci_HeldtoMaturitySecuritiesSoldSecurityRealizedLoss | [2] | $ 0gbci_HeldtoMaturitySecuritiesSoldSecurityRealizedLoss | [2] | $ 0gbci_HeldtoMaturitySecuritiesSoldSecurityRealizedLoss | [2]
[1] | The cost of each investment security sold is determined by specific identification.
[2] | The gain or loss on sale of held-to-maturity investment securities is solely due to securities that were partially or wholly called.

X
- Definition The total amount of amortized cost of the securities sold, categorized neither as held-to-maturity nor trading securities, during the reporting period. + References No definition available. + Details
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Balance Type: | debit
Period Type: | duration

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- Definition The total amount of amortized cost of the securities sold, categorized neither as available-for-sale nor trading securities, during the reporting period. + References No definition available. + Details
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- Definition The total amount of proceeds received for the sale of securities categorized as held-to-maturity during the reporting period. The sale of held-to-maturity securities includes proceeds from maturities, prepayments and calls. + References No definition available. + Details
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- Definition This item represents the gain realized on the sale of held-to-maturity securities disposed of during the period. + References No definition available. + Details
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- Definition This item represents the loss realized on the sale of held-to-maturity securities disposed of during the period. Additionally, this item may include, if not separately presented, any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt securities which are being or have been recognized in earnings during the period. + References No definition available. + Details
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- Definition This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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- Definition The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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- Definition This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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- Definition This item represents the gain (loss) realized on the sale of held-to-maturity securities disposed of during the period. Additionally, this item may include, if not separately presented, any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt securities which are being or have been recognized in earnings during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27405-111563 + Details
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