Compensation - Summary of Compensation Expense Related to Amortization of LPUs Held by BGC Employees (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LPU amortization	$ 18,455	$ 4,213	$ 54,288	$ 17,947
Stated vesting schedule [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LPU amortization	18,461	4,330	53,197	18,522
Post-termination payout [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LPU amortization	$ (6)	$ (117)	$ 1,091	$ (575)