Condensed Consolidated Statements of Changes in Equity (Parenthetical) - shares	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Repurchase of Class A common stock, shares	950,494
Forfeitures of restricted Class A common stock, shares	36,000
Class A Common Stock [Member]
Equity-based compensation, shares	437,018	637,719
Grant of exchangeability and redemption of limited partnership interests, shares	5,361,137	8,705,906
Issuance of Class A common stock (net of costs), shares	33,616	2,004,533
Repurchase of Class A common stock, shares	950,494	10,823,942
Forfeitures of restricted Class A common stock, shares	35,598	59,317
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partner units, units		624,762
Issuance of Class A common stock for acquisitions/Mergers, shares	1,341,783
Convertible notes, rate		4.50%
Convertible Notes, shares		6,909
Partnership Units [Member] | Class A Common Stock [Member]
Redemption of FPUs	276,389	460,690
Smith Mack [Member] | Class A Common Stock [Member]
Issuance of Class A common stock for acquisitions/Mergers, shares		1,853,259
GFI Group Inc. [Member] | Class A Common Stock [Member]
Issuance of Class A common stock for acquisitions/Mergers, shares		23,481,192