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
	Sep. 30, 2021	Aug. 31, 2021	Jul. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share issuances:
Redemptions/Exchanges of limited partnership interests (in shares)				8,446,093			1,746,526	49,033,008	5,820,715
Restricted stock forfeitures (in shares)				(56,000)			0	(140,000)	0
Treasury stock repurchases (in shares)	(8,534,000)	(5,899,000)	(10,000,000)		(16,542,000)	(965,000)		(41,940,000)
Class A Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at beginning of period (in shares)			348,795,000	348,795,000		323,018,000	313,323,000	323,018,000	307,915,000
Share issuances:
Redemptions/Exchanges of limited partnership interests (in shares)				8,446,000			1,747,000	49,033,000	5,821,000
Vesting of RSUs (in shares)				213,000			115,000	1,941,000	915,000
Acquisitions (in shares)				343,000			42,000	1,130,000	327,000
Other issuances of BGC Class A common stock (in shares)				100,000			90,000	366,000	339,000
Restricted stock forfeitures (in shares)				(56,423)			0	(140,188)	0
Treasury stock repurchases (in shares)				(24,432,718)			(2,259)	(41,939,724)	(2,259)
Shares outstanding at end of period (in shares)	333,408,000			333,408,000	348,795,000		315,315,000	333,408,000	315,315,000