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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Per Share Operating Performance:    
Net asset value, beginning of period $ 9.29 $ 8.69
Total income (loss) (1.7) (2.03)
Total expenses (0.02) (0.02)
Net increase (decrease) in net asset value (1.72) (2.05)
Net asset value, end of period $ 7.57 $ 6.64
Total Return (18.51%) (23.59%)
Ratios to Average Net Assets    
Total income (loss) (18.65%) (23.33%)
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.73% 0.62%
Net income (loss) (18.97%) (23.64%)
[1] Annualized.