Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Detail) - shares	1 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	May 01, 2018	Apr. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Share issuances:
Treasury stock repurchases	(6,543)	(281,339)	(438,657)			(726,539)
Forfeitures of restricted Class A common stock						(145,000)	(36,000)
Class A Common Stock [Member]
Class of Stock [Line Items]
Shares outstanding at beginning of period		279,279,244	279,279,244	279,279,244	248,466,750	256,968,372	244,869,624	244,869,624
Share issuances:
Issuance of Class A common stock for general corporate purposes				7,000,000		24,923,714
Exchanges of limited partnership interests				4,687,831	1,950,465	7,475,021	5,361,137	9,179,295
Compensation awards						979,344
Acquisition related issuances				687,376	968,705	1,004,472	1,341,783
Vesting of restricted stock units (RSUs)				100,790	80,629	381,674	437,018
Other issuances of Class A common stock				25,771	23,674	48,415	33,616
Treasury stock repurchases				(726,539)	(397,539)	(726,539)	(950,494)	(1,386,769)
Forfeitures of restricted Class A common stock				(144,630)	(35,598)	(144,630)	(35,598)	(183,246)
Shares outstanding at end of period	290,909,843			290,909,843	251,057,086	290,909,843	251,057,086	256,968,372