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Limited Partnership Interests in BGC Holdings and Newmark Holdings - Additional Information (Detail)
 3 Months Ended 12 Months Ended
Nov. 30, 2018
Nov. 23, 2018
shares
Jun. 05, 2015
shares
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
shares
Mar. 31, 2018
Dec. 31, 2019
Noncontrolling Interest [Abstract]                  
Number of operating partnerships             2    
Limited partnership interest contribution ratio             45.45%    
Limited partnership interests Exchange ratio,             95.58%    
Payout period for post-termination awards             Four equal yearly installments    
Percentage to preferred units             0.6875%    
Distribution ratio 46.3895%             45.45%  
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   712,907 34,649,693       23,600,000    
Cantor Rights to Purchase Exchangeable Units [Member]                  
Noncontrolling Interest [Abstract]                  
Number of shares, right to exchange from Class A to Class B common stock   10,323,366         23,613,420    
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     34,649,693       23,600,000    
Scenario Forecast [Member]                  
Noncontrolling Interest [Abstract]                  
Percentage to preferred units       0.6875% 0.6875% 0.6875%     2.75%