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Related Party Transactions - BGC's 2018 Investment in Newmark Holdings (Details) - USD ($)
3 Months Ended
Mar. 07, 2018
Sep. 26, 2018
Nov. 06, 2018
BGC Partners LP and its Operating Subsidiaries | Newmark Units      
Related Party Transaction      
Purchase of units (units) 16,600,000    
Purchase value of units $ 242,000,000.0    
Closing price per share (in dollars per share) $ 14.57    
Limited partners capital account units held by BGC (in shares) 14,800,000    
BGC Partners LP and its Operating Subsidiaries | Newmark Units | Newmark OpCo      
Related Party Transaction      
Limited partners capital account units held by BGC (in shares) 7,000,000.0    
Cantor      
Related Party Transaction      
Fees for services   $ 4,000,000.0  
Cantor | 6.125% Senior Notes      
Related Party Transaction      
Debt instrument face amount     $ 550,000,000.0
Stated interest rate (in percentage)     6.125%
Debt instrument, fee amount     $ 800,000