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Derivatives - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
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