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		12 Months Ended
	Sep. 30, 2017	Aug. 31, 2017	Jul. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Share issuances:
Treasury stock repurchases	(219,238)	(56,827)	(19,806)		(397,539)	(552,955)		(1,246,365)
Forfeitures of restricted Class A common stock								(107,000)	(59,000)
Class A Common Stock [Member]
Class of Stock [Line Items]
Shares outstanding at beginning of period			251,057,086	251,057,086		244,869,624	241,292,033	244,869,624	219,063,365	219,063,365
Share issuances:
Exchanges of limited partnership interests				1,434,050			2,947,876	6,795,187	6,269,630	8,705,906
Issuance of Class A common stock for general corporate purposes									1,648,000
Vesting of restricted stock units (RSUs)				42,295			81,873	479,313	569,344
Acquisitions				82,907			125,111	1,424,690	24,854,144
Other issuances of Class A common stock				11,874			250,219	45,490	292,524
Treasury stock repurchases				(295,871)			(1,341,947)	(1,246,365)	(9,326,182)	(10,823,942)
Forfeitures of restricted Class A common stock				(71,251)			(43,657)	(106,849)	(59,317)	(59,317)
Shares outstanding at end of period	252,261,090			252,261,090	251,057,086		243,311,508	252,261,090	243,311,508	244,869,624