Stock Transactions and Unit Redemptions - Schedule of Share Repurchase Activity (Detail) - Class A Common Stock - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Shares repurchased (in shares)	4,568,002				4,568,002	50,000
Treasury stock repurchases (in shares)	625,597	2,279,373	1,613,032	0	4,518,002	50,000
Average Price Paid per Unit or Share (in dollars per share)	$ 12.97	$ 8.81	$ 8.61	$ 0	$ 9.32	$ 9.73
Total Number of Shares Repurchased as Part of Publicly Announced Program	625,597	2,279,373	1,613,032	0	4,568,002	50,000
Approximate Dollar Value of Units and Shares That May Yet Be Repurchased/ Purchased Under the Plan	$ 157,413				$ 157,413	$ 199,514