Financial Highlights - Schedule of Financial Highlights (Parenthetical) (Detail) - USD ($)	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2013
Schedule Of Financial Highlights [Line Items]
Weighted average units outstanding	23,074,683	11,072,604			20,576,797	9,341,204
Contributions from Sponsor			$ 31,750	$ 51,034			$ 51,034
Total return based on net asset value					6.50%	6.30%
Ratio of net investment income/(loss) to average net assets					8.51%	8.32%
Ratio of net operating expenses to average net assets					2.27%	2.71%
Responsibility Agreement [Member]
Schedule Of Financial Highlights [Line Items]
Total return based on net asset value					4.38%	3.98%
Ratio of net investment income/(loss) to average net assets					5.73%	5.25%
Ratio of net operating expenses to average net assets					5.06%	5.77%