Derivatives - Offsetting of Derivative Instruments (Parenthetical) (Detail) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Offsetting [Abstract]
Additional balance in gross amounts not offset	$ 0	$ 0

Derivative additional balance in gross amounts not offset.

No definition available.

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