Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Detail) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share issuances:
Treasury stock repurchases		(233,172)
Class A Common Stock [Member]
Class of Stock [Line Items]
Shares outstanding at beginning of period	307,915,000	291,475,000
Share issuances:
Redemptions/Exchanges of limited partnership interests	2,105,401	1,820,588
Vesting of RSUs	697,000	240,000
Acquisitions	270,000	18,217
Other issuances of BGC Class A common stock	72,000	61,000
Treasury stock repurchases	0	(233,000)
Shares outstanding at end of period	311,059,000	293,382,000