Stock Transactions and Unit Redemptions - Gross Unit Redemptions and Share Repurchases of Class A Common Stock (Parenthetical) (Detail) - USD ($)	1 Months Ended	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Mar. 31, 2018
Class of Stock [Line Items]
Average price for redemption of limited partnership units		$ 6.00	$ 14.34
Redemptions, number of units		1,202,948
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired	233,172	233,172
Aggregate purchase price of Class A common stock		$ 1,236,000
Average price of Class A common stock	$ 5.30	$ 5.30
Class A Common Stock [Member]
Class of Stock [Line Items]
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired		233,172	0
Aggregate purchase price of Class A common stock		$ 1,200,000
Average price of Class A common stock		$ 5.30
Class A Common Stock [Member] | Controlled Equity Offerings [Member]
Class of Stock [Line Items]
Redemptions, number of units		900,000
Limited Partnership [Member]
Class of Stock [Line Items]
Redeemed limited partnership units		1,200,000	1,700,000
Aggregate redemption price of limited partnership units		$ 7,200,000	$ 24,600,000
Other Ownership Interest [Member]
Class of Stock [Line Items]
Redeemed limited partnership units		2,300	5,000
Aggregate redemption price of limited partnership units		$ 14,100	$ 74,000
Average price for redemption of founding/working partner units		$ 6.11	$ 13.67