Compensation - Activity Associated with LPU Awarded to BGC Employees (Detail) - LPUs [Member] - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	79,729,000	58,323,000	50,835,000
Number of Units, Granted	47,916,000	36,821,000	20,309,000
Number of Units, Redeemed/exchanged units	(21,110,000)	(14,908,000)	(7,283,000)
Number of Units, Forfeited units	(4,128,000)	(507,000)	(5,538,000)
Number of Units, Balance outstanding at end of period	102,407,000	79,729,000	58,323,000
Newmark Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	22,113,000	26,510,000
Number of Units, Granted	662,000	2,694,000	26,510,000
Number of Units, Redeemed/exchanged units	(1,024,000)	(7,032,000)
Number of Units, Forfeited units	(7,144,000)	(59,000)
Number of Units, Balance outstanding at end of period	14,607,000	22,113,000	26,510,000