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Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019 | Dec. 31, 2017 | Dec. 31, 2014
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Commitments and Contingencies [Line Items] |  |  |  |
Commitment to purchase alternative investment |  | $ 10,000 | $ 50,000 | $ 50,000
Future Funding Commitments | $ 31,214 | $ 31,720 |  |
European Non-Performing Loan Fund, LP |  |  |  |
Commitments and Contingencies [Line Items] |  |  |  |
Funded commitment amount | 35,800 |  |  |
Distressed Debt Fund, LP |  |  |  |
Commitments and Contingencies [Line Items] |  |  |  |
Funded commitment amount | 33,000 |  |  |
Unfunded Commitments | European Non-Performing Loan Fund, LP |  |  |  |
Commitments and Contingencies [Line Items] |  |  |  |
Future Funding Commitments | 14,200 |  |  |
Unfunded Commitments | Distressed Debt Fund, LP |  |  |  |
Commitments and Contingencies [Line Items] |  |  |  |
Future Funding Commitments | $ 17,000 |  |  |

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- Definition Commitment To Purchase Alternative Investment + References No definition available. + Details
Name: | gbli_CommitmentToPurchaseAlternativeInvestment
Namespace Prefix: | gbli_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments And Contingencies [Line Items] + References No definition available. + Details
Name: | gbli_CommitmentsAndContingenciesLineItems
Namespace Prefix: | gbli_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Funded Commitment Amount + References No definition available. + Details
Name: | gbli_FundedCommitmentAmount
Namespace Prefix: | gbli_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Future Funding Commitments + References No definition available. + Details
Name: | gbli_FutureFundingCommitments
Namespace Prefix: | gbli_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=gbli_EuropeanNonPerformingLoanFundLimitedPartnershipMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=gbli_DistressedDebtFundLimitedPartnershipMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_OtherCommitmentsAxis=gbli_UnfundedCommitmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |