XML 117 R52.htm IDEA: XBRL DOCUMENT v3.20.1
Limited Partnership Interests in BGC Holdings and Newmark Holdings - Additional Information (Detail) - shares
 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
Noncontrolling Interest [Abstract]                
Number of operating partnerships           2    
Limited partnership interest contribution ratio           45.45%    
Limited partnership interests Exchange Ratio           94.61%    
Payout period for post-termination awards           four equal yearly installments    
Percentage to preferred units           0.6875%    
Distribution ratio           46.3895% 45.45%  
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 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              
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%