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Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Detail) - shares
 3 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share issuances:        
Treasury stock repurchases (2,000) (2,000)    
Forfeitures of restricted BGC Class A common stock   0 (22,000)  
Class A Common Stock [Member]        
Class of Stock [Line Items]        
Shares outstanding at beginning of period   307,915,000 291,475,000  
Share issuances:        
Redemptions/exchanges of limited partnership interests   13,190,311 15,008,431 18,287,721
Vesting of RSUs   1,134,000 435,000  
Acquisitions   391,000 1,039,000  
Other issuances of BGC Class A common stock   345,000 213,000  
Issuance of BGC Class A common stock for general corporate purposes   45,000    
Treasury stock repurchases   (2,259) (233,172) (788,788)
Forfeitures of restricted BGC Class A common stock     (22,000) (231,602)
Shares outstanding at end of period   323,018,000 307,915,000 291,475,000