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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Per Share Operating Performance:      
Net asset value, beginning of year $ 17 $ 26.59 $ 42.83
Total income (loss) 3.52 (9.41) (15.85)
Net expenses (0.13) (0.18) (0.39)
Net increase (decrease) in net asset value 3.39 (9.59) (16.24)
Net asset value, end of year $ 20.39 $ 17 $ 26.59
Total Return 19.94% (36.07%) (37.92%)
Ratios to Average Net Assets      
Total income (loss) 27.09% (37.86%) (34.74%)
Management fees 0.60% 0.60% 0.60%
Expenses excluding management fees 0.12% 0.20% 0.33%
Net income (loss) 26.36% (38.65%) (35.66%)