Compensation - Compensation Expense Related to Restricted Stock Unit Held by BGC Employees (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Compensation Related Costs [Abstract]
RSU amortization	$ 2,916	$ 1,582

Share based compensation arrangement by share based payment award restricted stock units amortization.

No definition available.

Name:	bgcp_ShareBasedCompensationArrangementByShareBasedPaymentAwardRestrictedStockUnitsAmortization
Namespace Prefix:	bgcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration