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Compensation - Summary of Compensation Expense Related to Amortization of LPUs Held by BGC Employees (Detail) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
LPU amortization | $ 17,094 | $ 16,309
Stated vesting schedule [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
LPU amortization | 17,074 | 16,277
Post-termination payout [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
LPU amortization | $ 20 | $ 32

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- Definition Share based compensation arrangement by share based payment award limited partners capital units amortization. + References No definition available. + Details
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Namespace Prefix: | bgcp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PlanNameAxis=bgcp_StatedVestingScheduleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PlanNameAxis=bgcp_PostTerminationPayoutMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |