Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Detail) - shares	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share issuances:
Treasury stock repurchases		(233,172)	(8,263)	(233,172)	(734,802)
Forfeitures of restricted BGC Class A common stock			(49,189)	(22,000)	(193,819)
Class A Common Stock [Member]
Class of Stock [Line Items]
Shares outstanding at beginning of period	297,871,034	291,474,768	290,909,843	291,474,768	256,968,372
Share issuances:
Redemptions/Exchanges of limited partnership interests	4,001,618		2,474,658	9,701,674	9,949,679
Vesting of RSUs	61,851		82,345	383,357	464,019
Acquisition related issuances	348,797		78,678	848,246	1,083,150
Compensation awards					979,344
Other issuances of BGC Class A common stock	53,810		23,767	162,237	72,182
Issuance of BGC Class A common stock for general corporate purposes					24,923,714
Treasury stock repurchases	0		(8,263)	(233,172)	(734,802)
Forfeitures of restricted BGC Class A common stock	(22,046)		(49,189)	(22,046)	(193,819)
Shares outstanding at end of period	302,315,064		293,511,839	302,315,064	293,511,839