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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Per Share Operating Performance:      
Net asset value, beginning of year $ 13.69 $ 18.32 $ 17.23
Total income (loss) (3.86) (4.33) 1.32
Net expenses (0.11) (0.30) (0.23)
Net increase (decrease) in net asset value (3.97) (4.63) 1.09
Net asset value, end of year $ 9.72 $ 13.69 $ 18.32
Total Return (29.00%) (25.27%) 6.33%
Ratios to Average Net Assets      
Total income (loss) (27.80%) (19.76%) 13.23%
Management fees 0.75% 0.75% 0.75%
Total expenses excluding management fees 0.71% 0.85% 0.51%
Expenses waived (0.56%) (0.67%) (0.30%)
Net expenses excluding management fees 0.15% 0.18% 0.21%
Net income (loss) (28.70%) (20.70%) 12.27%