Compensation - Activity Associated with Limited Partnership Units Awarded to BGC Employees (Detail) - LPUs [Member] - shares shares in Thousands	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, Redeemed	17,709	21,343	16,527
BGC Holdings LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Redeemed	17,209	18,227	15,460
Newmark Holdings LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Redeemed	500	3,116	1,067