Compensation - Units Redeemed in Connection with Issuance of Class A Common Stock (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award
Number of units redeemed (in shares)	60,312	151,160	419,797	588,160
BGC Units
Share-based Compensation Arrangement by Share-based Payment Award
Number of units redeemed (in shares)	34,893	87,566	225,725	379,191
Newmark Units
Share-based Compensation Arrangement by Share-based Payment Award
Number of units redeemed (in shares)	25,419	63,594	194,072	208,969