Stock Transactions and Unit Redemptions - Unit Redemption and Share Repurchase Activity (Detail) - USD ($)	1 Months Ended			3 Months Ended		6 Months Ended
	Jun. 30, 2017	May 31, 2017	Apr. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2017
Stock Transactions Parenthetical Disclosures [Abstract]
Redemptions, Number of Units				1,908,867	2,927,758	4,800,000
Repurchases, Number of Shares	27,634	121,023	248,882		552,955	950,494
Total Redemptions and Repurchases						5,787,119
Redemptions, Average Price Paid per Unit				$ 11.14	$ 10.84
Repurchases, Average Price Paid per Share	$ 12.21	$ 11.57	$ 10.99		$ 11.07	$ 11.14
Total Redemptions and Repurchases						$ 10.99
Total Redemptions and Repurchases, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan	$ 253,618,546			$ 253,618,546		$ 253,618,546