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Limited Partnership Interests in BGC Holdings and Newmark Holdings - Additional Information (Detail) - shares
shares in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Nov. 23, 2018
Jun. 05, 2015
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2018
Dec. 31, 2020
Dec. 31, 2019
Noncontrolling Interest [Abstract]                  
Number of operating partnerships               2  
Limited partnership interest contribution ratio               45.45%  
Limited partnership interests Exchange Ratio               93.79%  
Payout period for post-termination awards               four equal yearly installments  
Percentage to preferred units     0.6875% 0.6875% 0.6875% 0.6875%   2.75%  
Distribution ratio             45.45% 46.3895%  
Common stock exchangeable basis               one-for-one basis  
Newmark [Member]                  
Noncontrolling Interest [Abstract]                  
Common stock exchangeable basis               one-for-one  
CF Group Management, Inc. [Member]                  
Noncontrolling Interest [Abstract]                  
Number of shares, right to exchange from Class A to Class B common stock 0.7 34.6             23.6
Cantor Rights to Purchase Exchangeable Units [Member]                  
Noncontrolling Interest [Abstract]                  
Number of shares, right to exchange from Class A to Class B common stock 10.3                
Common stock exchangeable basis               one-for-one  
Cantor Rights to Purchase Exchangeable Units [Member] | Maximum [Member]                  
Noncontrolling Interest [Abstract]                  
Number of shares, right to exchange from Class A to Class B common stock   34.6             23.6