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Compensation - Units Redeemed in Connection with Issuance of Class A Common Stock (Details) - shares
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award    
Number of units redeemed (in shares) 686,544 291,721
BGC Units    
Share-based Compensation Arrangement by Share-based Payment Award    
Number of units redeemed (in shares) 46,605 149,280
Newmark Units    
Share-based Compensation Arrangement by Share-based Payment Award    
Number of units redeemed (in shares) 639,939 142,441