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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Per Share Operating Performance:
Net asset value, beginning of period	$ 21.05	$ 20.39	$ 17.00
Total income (loss)	(3.04)	0.82	3.52
Total expenses	(0.19)	(0.16)	(0.13)
Net increase (decrease) in net asset value	(3.23)	0.66	3.39
Net asset value, end of period	$ 17.82	$ 21.05	$ 20.39
Total Return	(15.34%)	3.24%	19.94%
Ratios to Average Net Assets
Total income (loss)	(6.16%)	2.46%	27.09%
Management fees	0.60%	0.60%	0.60%
Expenses excluding management fees	0.22%	0.26%	0.12%
Net income (loss)	(6.98%)	1.60%	26.36%