Compensation - Summary of Outstanding LPUs Held by BGC Employees with Stated Vesting (Detail) - LPUs - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate estimated grant date fair value	$ 185,023	$ 201,239
BGC Holdings LPUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate estimated grant date fair value	43,816	44,529
Newmark Holdings LPUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate estimated grant date fair value	$ 261	$ 353