Net Asset Value and Financial Highlights - Schedule of Net Asset Value and Financial Highlights (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016
Investment operations:
Total Return, at net asset value	7.43%	(9.09%)	(0.52%)
Ratio/Supplemental Data:
Net assets, end of period	$ 127,835,843	$ 154,855,001	$ 154,956,309	$ 114,110,261	$ 159,102,078	$ 160,409,438	$ 219,660,595
Ratios to average net assets of:
Net investment income	1.30%	0.94%	0.01%
Expenses, prior to expense waivers	0.85%	0.85%	0.85%
Expenses, net of expense waivers	0.75%	0.75%	0.75%
Limited Units [Member]
Net Asset Value
Net asset value per Limited Share, beginning of period	$ 17.50	$ 19.25	$ 19.35
Investment operations:
Net realized and unrealized loss	1.07	(1.93)	(0.10)
Net investment income	0.23	0.18	0.00
Net increase (decrease) in net assets from operations	1.30	(1.75)	(0.10)
Net asset value per Limited Share, end of period	$ 18.80	$ 17.50	$ 19.25