Derivatives - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Replacement cost of contracts in a gain position	$ 4.2	$ 0.9

The replacement cost of derivative contracts in a gain or asset position.

No definition available.

Name:	bgcp_DerivativeContractInGainPositionReplacementCost
Namespace Prefix:	bgcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant