Compensation - Activity Associated with Limited Partnership Units Awarded to BGC Employees (Detail) - LPUs [Member] - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Redeemed	571	5,272	4,764	5,963
BGC Holdings LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Redeemed	471	5,101	4,473	5,621
Newmark Holdings LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Redeemed	100	171	291	342