FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Per Share Operating Performance:
Net asset value, beginning of year	$ 11.75	$ 9.72	$ 13.69
Total income (loss)	(2.4)	2.12	(3.86)
Net expenses	(0.09)	(0.09)	(0.11)
Net increase (decrease) in net asset value	(2.49)	2.03	(3.97)
Net asset value, end of year	$ 9.26	$ 11.75	$ 9.72
Total Return	(21.19%)	20.88%	(29.00%)
Ratios to Average Net Assets
Total income (loss)	(29.05%)	21.94%	(27.80%)
Management fees	0.75%	0.75%	0.75%
Total expenses excluding management fees	0.74%	0.90%	0.71%
Expenses waived	(0.59%)	(0.75%)	(0.56%)
Net expenses excluding management fees	0.15%	0.15%	0.15%
Net income (loss)	(29.95%)	21.04%	(28.70%)