Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Detail) - shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share issuances:
Exchange from Class A to Class B common stock			(11,036,273)
Treasury stock repurchases	(233,000)	(233,000)
Forfeitures of restricted Class A common stock		(22,000)	(232,000)
Class A Common Stock [Member]
Class of Stock [Line Items]
Shares outstanding at beginning of period	291,475,000	291,475,000	256,968,000
Share issuances:
Redemption/exchanges of limited partnership interests		15,008,431	18,287,721	9,179,295
Issuance of Class A common stock for general corporate purposes			24,924,000
Deferred stock awards			979,000
Vesting of restricted stock units (RSUs)		435,000	528,000
Acquisitions		1,039,000	1,744,000
Other issuances of Class A common stock		213,000	101,000
Exchange from Class A to Class B common stock			(11,036,000)
Treasury stock repurchases		(233,172)	(788,788)	(1,386,769)
Forfeitures of restricted Class A common stock		(22,000)	(231,602)	(183,246)
Shares outstanding at end of period		307,915,000	291,475,000	256,968,000