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Condensed Consolidating Financial Information Provided in Connection with Outstanding Debt of Subsidiaries - Additional Information (Detail) | 9 Months Ended
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Sep. 30, 2020
GBLI Holdings, LLC |
Condensed Financial Statements, Captions [Line Items] |
Ownership percentage in subsidiary | 100.00%

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | srt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of subsidiary's or equity method investee's stock owned by parent immediately after all stock transactions. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=gbli_GBLIHoldingsLimitedLiabilityCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |