Stock Transactions and Unit Redemptions - Gross Unit Redemptions and Share Repurchases of Class A Common Stock (Detail) - USD ($)	3 Months Ended				9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Stock Transactions Parenthetical Disclosures [Abstract]
Redemptions, Number of Units	73,295	97,229	1,202,948	0	1,373,472	0
Repurchases, Number of Shares			233,172	8,263	233,172	734,802
Total Number of Units Redeemed or Shares Repurchased					1,606,644
Redemptions, Average Price Paid per Unit or Share	$ 5.24	$ 5.45	$ 6.00		$ 5.92
Repurchases, Average Price Paid per Unit or Shares			$ 5.30		5.30
Total Redemptions and Repurchases, Average Price Paid per Unit or Shares					$ 5.83
Total Redemptions and Repurchases, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan				$ 256,882,218		$ 256,882,218