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Compensation - Additional Information (Detail)
$ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
shares
Mar. 31, 2017
USD ($)
shares
Dec. 31, 2017
USD ($)
shares
Dec. 13, 2017
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exchange ratio standalone number of units | $ $ 0.7      
Notional value with post-termination pay-out amount | $ 139.2      
Aggregate estimated fair value of limited partnership units | $ 28.6      
Income allocated to limited partnership units | $ $ 4.1 $ 4.6    
Restricted stock unit, Conversion description Each RSU is settled in one share of BGC’s Class A common stock upon completion of the vesting period. Future RSU awards will be settled in one share of Newmark Class A common stock upon completion of the vesting period.      
Deferred cash compensation expense recognized | $ $ 1.1 0.2    
Liability for deferred cash compensation awards | $ $ 0.6   $ 0.4  
Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Limited partnership units outstanding 241,295   346,538  
Equity-based compensation expense | $ $ 0.3 $ 0.3    
Awards granted   300,000    
Aggregate estimated grant date fair values | $   $ 2.8    
Aggregate estimated grant date fair value of outstanding RSUs | $ 2.3   $ 3.3  
Unrecognized compensation expense related to unvested RSUs | $ 2.5      
Executives and Non-Executive Employees [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Notional value with post-termination pay-out amount | $ 21.6   $ 232.9  
Aggregate estimated fair value of limited partnership units | $ $ 39.2      
Number of unvested limited partnership units with post-termination pay-out     23,400,000  
Number of outstanding limited partnership units, unvested     13,200,000  
Limited Partnership Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Limited partnership units outstanding 33,011,995   29,413,143  
Aggregate estimated fair value of limited partnership units | $ $ 6.7   $ 14.3  
Compensation expense (benefit) before income tax related to limited partnership units | $ $ (8.7) 5.8    
Awards granted 3,767,619      
BGC Holdings, L.P. [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of limited partnership units exchangeable into shares 17,200,000   12,300,000  
Number of unvested limited partnership units with post-termination pay-out 13,800,000      
Number of outstanding limited partnership units, unvested 6,600,000      
BGC Holdings, L.P. [Member] | Executives and Non-Executive Employees [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of unvested limited partnership units with post-termination pay-out     23,400,000  
Number of outstanding limited partnership units, unvested     13,200,000  
BGC Holdings, L.P. [Member] | Limited Partnership Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Limited partnership units outstanding 64,337,628   64,708,915  
Number of limited partnership units granted exchangeability 1,500,000      
Equity-based compensation expense | $ $ 21.7 $ 6.0    
Newmark Holdings, L.P. [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of limited partnership units exchangeable into shares 8,100,000   5,600,000  
Number of unvested limited partnership units with post-termination pay-out 6,300,000      
Number of outstanding limited partnership units, unvested 3,000,000      
Newmark Holdings, L.P. [Member] | Executives and Non-Executive Employees [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Aggregate estimated fair value of limited partnership units | $ $ 2.1      
Number of unvested limited partnership units with post-termination pay-out 1,500,000   10,600,000  
Number of outstanding limited partnership units, unvested 1,100,000   6,000,000  
Newmark Holdings, L.P. [Member] | Limited Partnership Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of limited partnership units granted exchangeability 700,000 600,000    
Equity-based compensation expense | $ $ 0.0      
Maximum [Member] | Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted shares, restriction period   4 years    
Maximum [Member] | Limited Partnership Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted shares, restriction period 5 years      
Minimum [Member] | Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted shares, restriction period   2 years    
Minimum [Member] | Limited Partnership Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted shares, restriction period 3 years      
Class A Common Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Limited partnership units exchange ratio 1      
Class A Common Stock [Member] | Newmark Holdings, L.P. [Member] | Limited Partnership Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Limited partnership units outstanding 33,000,000   29,400,000  
Limited partnership units issued 3,800,000      
Newmark Equity Plan [Member] | Class A Common Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares registered to be delivered pursuant to awards granted       50,000,000
Newmark Equity Plan [Member] | Class A Common Stock [Member] | Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares authorized to be delivered pursuant to awards granted       400,000,000