Stock Transactions and Unit Redemptions - Schedule of Share Repurchase Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Class of Stock
Shares repurchased (in shares)	4,568,002				4,568,002		4,568,002
Class A Common Stock
Class of Stock
Shares repurchased (in shares)	4,568,002				4,568,002		4,568,002
Treasury stock repurchases (in shares)	0	0	0	2,279,373	0	3,892,405
Average price paid per unit or share (in dollars per share)	$ 0	$ 0	$ 0		$ 9.32		$ 9.32
Total Number of Shares Repurchased as Part of Publicly Announced Program	0	0	0		4,568,002		4,568,002
Approximate Dollar Value of Units and Shares That May Yet Be Repurchased/ Purchased Under the  Program	$ 157,413				$ 157,413		$ 157,413