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Derivatives - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Replacement cost of contracts in a gain position	$ 0.5	$ 1.9

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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