Compensation - Units Redeemed in Connection with Issuance of Class A Common Stock (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Number of units redeemed (in shares)	4,977,354	2,931,287	25,253,431
BGC Units
Share-based Compensation Arrangement by Share-based Payment Award
Number of units redeemed (in shares)	315,685	620,903	18,325,470
Newmark Units
Share-based Compensation Arrangement by Share-based Payment Award
Number of units redeemed (in shares)	4,661,669	2,310,384	6,927,961