Condensed Consolidated Statements of Changes in Equity (Parenthetical) - shares	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Repurchase of Class A common stock, shares	726,539
Forfeitures of restricted Class A common stock, shares	145,000
Class A Common Stock [Member]
Equity-based compensation, shares	381,674	570,944
Grant of exchangeability and redemption of limited partnership interests, shares	7,475,021	9,179,295
Issuance of Class A common stock (net of costs), shares	25,951,473	1,994,670
Repurchase of Class A common stock, shares	726,539	1,386,769
Forfeitures of restricted Class A common stock, shares	144,630	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	1,004,472
Partnership Units [Member] | Class A Common Stock [Member]
Redemption of FPUs	81,123	729,802
Smith Mack [Member] | Class A Common Stock [Member]
Issuance of Class A common stock for acquisitions, shares	1,004,472	1,923,854