Compensation - Activity Associated with Limited Partnership Units Awarded to BGC Employees (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Newmark Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, redeemed units	(100,000)	0	(300,000)	0
LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period			79,729,188
Number of Units, Granted			37,119,394
Number of Units, redeemed units			(12,913,487)
Number of Units, Forfeited units/other			(4,064,102)
Number of Units, Balance outstanding at end of period	99,870,993		99,870,993
LPUs [Member] | Newmark Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period			22,113,105
Number of Units, Granted			611,637
Number of Units, redeemed units			(592,362)
Number of Units, Forfeited units/other			(7,118,992)
Number of Units, Balance outstanding at end of period	15,013,388		15,013,388