Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Detail) - shares	1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share issuances:
Treasury stock repurchases	(2,000)	(2,259)		(2,259)	(233,172)
Forfeitures of restricted BGC Class A common stock		0	(22,000)	0	(22,000)
Class A Common Stock [Member]
Class of Stock [Line Items]
Shares outstanding at beginning of period	313,323,000	313,323,000	297,871,000	307,915,000	291,475,000
Share issuances:
Redemptions/Exchanges of limited partnership interests		1,746,526	4,001,618	5,820,715	9,701,674
Vesting of RSUs		115,000	62,000	915,000	383,000
Acquisitions		42,000	349,000	327,000	848,000
Other issuances of BGC Class A common stock		90,000	53,000	339,000	162,000
Treasury stock repurchases		(2,000)	0	(2,000)	(233,000)
Forfeitures of restricted BGC Class A common stock			(22,000)		(22,000)
Shares outstanding at end of period		315,315,000	302,315,000	315,315,000	302,315,000