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Derivatives - Additional Information (Detail) - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
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Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |
Replacement cost of contracts in a gain position | $ 5.2 | $ 0.9

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- Definition The replacement cost of derivative contracts in a gain or asset position. + References No definition available. + Details
Name: | bgcp_DerivativeContractInGainPositionReplacementCost
Namespace Prefix: | bgcp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration