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Consolidated Statements of Changes in Equity (Parenthetical) - $ / shares
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Repurchase of Class A common stock, shares 233,000    
Forfeitures of restricted Class A common stock, shares 22,000 232,000  
Exchange class A to class B common stock, shares   11,036,273  
Dividends declared per share of common stock $ 0.56 $ 0.72 $ 0.70
Dividends declared and paid per share of common stock $ 0.56 $ 0.72 $ 0.70
Class A Common Stock [Member]      
Equity-based compensation, shares 434,694 527,951 570,944
Grant of exchangeability and redemption of limited partnership interests, shares 15,008,431 18,287,721 9,179,295
Issuance of Class A common stock (net of costs), shares 212,711 26,003,424 1,994,670
Repurchase of Class A common stock, shares 233,172 788,788 1,386,769
Forfeitures of restricted Class A common stock, shares 22,000 231,602 183,246
Cantor purchase of Cantor units from BGC Holdings upon redemption of Founding/Working Partner Units, units     1,179,788
Issuance of Class A common stock and RSUs for acquisitions, shares 1,039,000 1,744,000  
Exchange class A to class B common stock, shares   11,036,000  
Forfeiture of Class A common stock, shares 22,046    
Class A Common Stock [Member] | Smith Mack [Member]      
Issuance of Class A common stock and RSUs for acquisitions, shares 1,039,452 1,743,963 1,923,854
Class A Common Stock [Member] | Partnership Units [Member]      
Redemption of FPUs 56,878 108,967 729,802