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Condensed Consolidated Statements of Changes in Equity (Parenthetical) - shares
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Forfeitures of restricted Class A common stock, shares 0  
Class A Common Stock [Member]    
Equity-based compensation, shares 280,884 570,944
Grant of exchangeability and redemption of limited partnership interests, shares 2,787,190 9,179,295
Issuance of Class A common stock (net of costs), shares 18,925,702 1,994,670
Repurchase of Class A common stock, shares 0 1,386,769
Forfeitures of restricted Class A common stock, shares   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 for acquisitions, shares 317,096  
Partnership Units [Member] | Class A Common Stock [Member]    
Redemption of FPUs 5,403 729,802
Smith Mack [Member] | Class A Common Stock [Member]    
Issuance of Class A common stock for acquisitions, shares 317,096 1,923,854