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Limited Partnership Interests - Additional Information (Detail) - USD ($)
shares in Millions, $ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2019
Dec. 31, 2019
Nov. 30, 2018
Sep. 26, 2018
Jun. 18, 2018
Noncontrolling Interest [Abstract]                  
Payout period for post-termination awards         four equal yearly installments        
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction     94.95%   94.95%        
Percentage to preferred units     0.6875% 0.6875%          
BGC Holdings, L.P. [Member]                  
Noncontrolling Interest [Abstract]                  
Limited partnership interest initial contribution ratio         45.45%        
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction             46.3895%    
CF Group Management, Inc. [Member]                  
Noncontrolling Interest [Abstract]                  
Number of shares, right to exchange from Class A to Class A or Class B common stock         23.0        
Cantor Rights to Purchase Exchangeable Units [Member]                  
Noncontrolling Interest [Abstract]                  
Number of shares, right to exchange from Class A to Class A or Class B common stock         23.0        
Scenario, Forecast [Member]                  
Noncontrolling Interest [Abstract]                  
Percentage to preferred units 0.6875% 0.6875%       2.75%      
Class A Common Stock [Member]                  
Noncontrolling Interest [Abstract]                  
Limited partnership units exchange ratio         0.9495        
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction             46.3895%    
Newmark OpCo [Member] | RBC [Member]                  
Noncontrolling Interest [Abstract]                  
Exchangeable preferred limited partnership units issued     $ 325.0   $ 325.0     $ 150.0 $ 175.0
Exchangeable preferred limited partnership units         $ 325.0