Compensation - Summary of Outstanding LPUs Held by BGC Employees with Stated Vesting (Detail) - LPUs [Member] - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate estimated grant date fair value	$ 195,689	$ 138,324
BGC Holdings LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate estimated grant date fair value	42,782	30,699
Newmark Holdings LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate estimated grant date fair value	$ 446	$ 1,171