Compensation - Units Redeemed in Connection with Issuance of Class A Common Stock (Details) - shares	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
Number of units redeemed (in shares)	64,764	257,091	356,485	346,091
BGC Units
Share-based Compensation Arrangement by Share-based Payment Award
Number of units redeemed (in shares)	38,552	232,625	187,832	291,625
Newmark Units
Share-based Compensation Arrangement by Share-based Payment Award
Number of units redeemed (in shares)	26,212	24,466	168,653	54,466