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Derivatives - Additional Information (Detail) - USD ($) | Dec. 31, 2020 | Dec. 31, 2019
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Derivative Instruments And Hedging Activities Disclosure [Abstract] |  |
Replacement costs of contracts in a gain position | $ 900,000 | $ 1,900,000
Additional balance in gross amounts not offset | $ 0 | $ 0

X
- Definition Derivative additional balance in gross amounts not offset. + References No definition available. + Details
Name: | bgcp_DerivativeAdditionalBalanceInGrossAmountsNotOffset
Namespace Prefix: | bgcp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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

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