Stock Transactions and Unit Redemptions - Gross Unit Redemptions and Share Repurchases of Class A Common Stock (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019
Stock Transactions Parenthetical Disclosures [Abstract]
Redemptions, Number of Units	35	73	97	1,203	1,408
Repurchases, Number of Shares				233	233
Total Number of Units Redeemed or Shares Repurchased					1,641
Redemptions, Weighted Average Price Paid per Unit or Share	$ 5.57	$ 5.24	$ 5.45	$ 6.00	$ 5.91
Repurchases, Weighted Average Price Paid per Unit or Shares				$ 5.30	5.30
Total Redemptions and Repurchases, Weighted Average Price Paid per Unit or Shares					$ 5.82
Total Redemptions and Repurchases, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Program	$ 256,691				$ 256,691