Stock Transactions and Unit Redemptions - Unit Redemption and Share Repurchase Activity (Detail) - USD ($)	1 Months Ended			3 Months Ended			9 Months Ended
	Sep. 30, 2018	Aug. 31, 2018	Jul. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018
Stock Transactions Parenthetical Disclosures [Abstract]
Redemptions, Number of Units				2,105,457	4,299,936	1,723,654	8,129,047
Repurchases, Number of Shares	5,574	696	1,993		726,539		734,802
Total Redemptions and Repurchases							8,863,849
Redemptions, Average Price Paid per Unit				$ 11.27	$ 13.10	$ 14.34	$ 12.89
Redemptions, Average Price Paid per Unit	$ 11.76	$ 11.85	$ 10.58		$ 13.37		13.35
Total Redemptions and Repurchases							$ 12.92
Total Redemptions and Repurchases, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan	$ 280,011,172			$ 280,011,172			$ 280,011,172