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Supplemental Non-Cash Investing and Financing Activities - Additional Information (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024
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Supplemental Cash Flow Elements [Line Items] |  |
Operating lease, right-of-use asset | $ 8,424 | $ 9,295

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- Definition Supplemental cash flow elements. + References No definition available. + Details
Name: | gbli_SupplementalCashFlowElementsLineItems
Namespace Prefix: | gbli_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of lessee's right to use underlying asset under operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479041/842-20-45-1 + Details
Name: | us-gaap_OperatingLeaseRightOfUseAsset
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant