Compensation - Activity Associated with LPU's Held by BGC Employees (Detail) - LPUs - shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Number of Units, Balance outstanding at beginning of period (in shares)	137,652,000	102,407,000	79,729,000
Number of Units, Granted (in shares)	34,093,000	50,269,000	47,916,000
Number of Units, Redeemed/exchanged units (in shares)	(58,832,000)	(14,642,000)	(21,110,000)
Number of Units, Forfeited units (in shares)	(798,000)	(382,000)	(4,128,000)
Number of Units, Balance outstanding at end of period (in shares)	112,115,000	137,652,000	102,407,000
Newmark Units
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Number of Units, Balance outstanding at beginning of period (in shares)	13,202,000	14,607,000	22,113,000
Number of Units, Granted (in shares)	0	0	662,000
Number of Units, Redeemed/exchanged units (in shares)	(1,881,000)	(1,300,000)	(1,024,000)
Number of Units, Forfeited units (in shares)	(270,000)	(105,000)	(7,144,000)
Number of Units, Balance outstanding at end of period (in shares)	11,051,000	13,202,000	14,607,000