Condensed Consolidated Statements of Changes in Equity (Parenthetical) - $ / shares	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Repurchase of Class A common stock, shares			734,802
Forfeitures of restricted Class A common stock, shares			194,000	107,000
Dividends declared per share of common stock	$ 0.18	$ 0.18	$ 0.54	$ 0.52
Dividends declared and paid per share of common stock	$ 0.18	$ 0.18	$ 0.54	$ 0.52
Class A Common Stock [Member]
Equity-based compensation, shares			464,019		570,944
Grant of exchangeability and redemption of limited partnership interests, shares	2,474,658	1,434,050	9,949,679	6,795,187	9,179,295
Issuance of Class A common stock (net of costs), shares			25,975,240		1,994,670
Repurchase of Class A common stock, shares	8,263	295,871	734,802	1,246,365	1,386,769
Forfeitures of restricted Class A common stock, shares	49,189	71,251	193,819	106,849	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	78,678	82,907	1,083,150	1,424,690
Partnership Units [Member] | Class A Common Stock [Member]
Redemption of FPUs			98,991		729,802
Smith Mack [Member] | Class A Common Stock [Member]
Issuance of Class A common stock for acquisitions, shares			1,083,150		1,923,854