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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LPU amortization	$ 19,862	$ 18,455	$ 53,696	$ 54,288
Stated vesting schedule
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LPU amortization	19,880	18,461	53,655	53,197
Post-termination payout
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LPU amortization	$ (18)	$ (6)	$ 41	$ 1,091