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Limited Partnership Interests in BGC Holdings and Newmark Holdings - Additional Information (Detail)
 3 Months Ended 12 Months Ended
Jun. 05, 2015
shares
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
shares
Dec. 31, 2018
Dec. 31, 2017
shares
Noncontrolling Interest [Abstract]              
Number of operating partnerships         2    
Limited partnership interest contribution ratio         45.45%    
Payout period for post-termination awards         four equal yearly installments    
Percentage to preferred units         0.6875%    
CF Group Management, Inc. [Member]              
Noncontrolling Interest [Abstract]              
Number of shares, right to exchange from Class A to Class B common stock 34,649,693            
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   34,649,693
Scenario Forecast [Member]              
Noncontrolling Interest [Abstract]              
Percentage to preferred units   0.6875% 0.6875% 0.6875%   2.75%