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Debt Securities - Gain or Loss on Sale (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Available-for-sale |  |  |  |
Proceeds from sales and calls of debt securities |  | $ 240,521 | $ 928,710 | $ 265,587
Gross realized gains | [1] | 1,400 | 18,936 | 443
Gross realized losses | [1] | (262) | (4,513) | (455)
Held-to-maturity |  |  |  |
Proceeds from calls of debt securities |  | 32,735 | 58,750 | 79,000
Gross realized gains | [1] | 1 | 2 | 101
Gross realized losses | [1] | $ 0 | $ (10) | $ (1,202)
[1] | The gain or loss on the sale or call of each debt security is determined by the specific identification method.

X
- 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
Name: | gbci_HeldToMaturitySecuritiesGrossRealizedGainsLossesSaleProceeds
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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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
Name: | gbci_HeldToMaturitySecuritiesSoldSecurityRealizedGain
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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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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Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Proceeds From Sales And Calls Of Debt Securities, Available-For-sale + References No definition available. + Details
Name: | gbci_ProceedsFromSalesAndCallsOfDebtSecuritiesAvailableForSale
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27357-111563 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleRealizedGain
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27357-111563 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
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Data Type: | xbrli:stringItemType
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Period Type: | duration