Stock Transactions and Unit Redemptions - The Unit Redemptions and Share Repurchases of Class A Common Stock (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Class of Stock [Line Items]
Weighted-average price for redemption of limited partnership units		$ 2.81	$ 5.19	$ 3.02	$ 5.94
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired	2,000	2,259		2,259	233,172
Aggregate purchase price of Class A common stock				$ 6	$ 1,236
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued for redemption/cancellation of units		500,000	2,800,000	2,600,000	5,200,000
Common stock, shares issued for exchange of units		1,300,000	1,200,000	3,100,000	3,600,000
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired		2,000	0	2,000	233,000
Aggregate purchase price of Class A common stock		$ 6		$ 6	$ 1,200
Weighted-average price of Class A common stock		$ 2.58		$ 2.58	$ 5.30
Limited Partnership [Member]
Class of Stock [Line Items]
Redeemed limited partnership units		1,500,000	36,000	1,800,000	1,300,000
Aggregate redemption price of limited partnership units		$ 4,100	$ 188	$ 5,400	$ 7,900
Other Ownership Interest [Member]
Class of Stock [Line Items]
Redeemed limited partnership units		27,000	37,000	28,000	42,000
Aggregate redemption price of limited partnership units		$ 73	$ 196	$ 77	$ 226
Weighted-average price for redemption of founding/working partner units		$ 2.74	$ 5.28	$ 2.75	$ 5.35