Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Detail) - shares	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share issuances:
Treasury stock repurchases		(233,172)	(726,539)	(233,172)	(726,539)
Forfeitures of restricted Class A common stock			(144,630)	0	(144,630)
Class A Common Stock [Member]
Class of Stock [Line Items]
Shares outstanding at beginning of period	293,382,378	291,474,768	279,279,244	291,474,768	256,968,372
Share issuances:
Redemptions/Exchanges of limited partnership interests	3,879,468		4,687,831	5,700,056	7,475,021
Vesting of RSUs	81,171		100,790	321,506	381,674
Acquisition related issuances	481,232		687,376	499,449	1,004,472
Compensation awards					979,344
Other issuances of Class A common stock	46,785		25,771	108,427	48,415
Issuance of Class A common stock for general corporate purposes			7,000,000		24,923,714
Treasury stock repurchases	0		(726,539)	(233,172)	(726,539)
Forfeitures of restricted Class A common stock			(144,630)		(144,630)
Shares outstanding at end of period	297,871,034	293,382,378	290,909,843	297,871,034	290,909,843