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, 2019	Jun. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan			$ 165,500
Class A Common Stock [Member]
Class Of Stock [Line Items]
Balance at beginning of period, Total Number of Shares Repurchased			50,000
Repurchase of common stock, shares	2,279,373	1,613,032	3,892,405
Balance at Ending of period, Total Number of Shares Repurchased	3,942,405		3,942,405	50,000
Repurchase, Average Price Paid per Unit	$ 8.81	$ 8.61	$ 8.74	$ 9.73
Total Number of Shares Purchased as Part of Publicly Announced Program	2,279,373	1,613,032	3,942,405	50,000
Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan	$ 165,530	$ 185,618	$ 165,530	$ 199,514