Stock Transactions and Unit Redemptions - Gross Unit Redemptions and Share Repurchases of Class A Common Stock (Detail) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Stock Transactions Parenthetical Disclosures [Abstract]
Redemptions, Number of Units	97,229	1,202,948		1,300,177
Repurchases, Number of Shares		233,172	726,539	233,172	726,539
Total Number of Units Redeemed or Shares Repurchased				1,533,349
Redemptions, Average Price Paid per Unit or Share	$ 5.45	$ 6.00		$ 5.96
Repurchases, Average Price Paid per Unit or Shares		$ 5.30		5.30
Total Redemptions and Repurchases, Average Price Paid per Unit or Shares				$ 5.86
Total Redemptions and Repurchases, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan	$ 257,265,928			$ 257,265,928