Stock Transactions and Unit Redemptions - Gross Unit Redemptions and Share Repurchases of Class A Common Stock (Parenthetical) (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Weighted-average price for redemption of limited partnership units		$ 5.92	$ 12.61
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired	233,000	233,000
Aggregate purchase price of Class A common stock		$ 1,236,000	$ 10,439,000	$ 16,773,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued for redemption/cancellation of units		10,100,000	6,800,000
Common stock, shares issued for exchange of units		4,400,000	2,600,000
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired		233,172	788,788	1,386,769
Aggregate purchase price of Class A common stock		$ 1,200,000	$ 10,400,000
Weighted-average price of Class A common stock		$ 5.30	$ 13.23
Limited Partnership [Member]
Class of Stock [Line Items]
Redeemed limited partnership units		1,400,000	9,200,000
Aggregate redemption price of limited partnership units		$ 8,000,000.0	$ 116,500,000
Other Ownership Interest [Member]
Class of Stock [Line Items]
Redeemed limited partnership units		56,900	108,900
Aggregate redemption price of limited partnership units		$ 320,600	$ 1,400,000
Weighted-average price for redemption of founding/working partner units		$ 5.64	$ 12.86