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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Per Share Operating Performance:    
Net asset value, beginning of period $ 9.29 $ 8.69
Total income (loss) (2.68) (0.30)
Total expenses (0.07) (0.06)
Net increase (decrease) in net asset value (2.75) (0.36)
Net asset value, end of period $ 6.54 $ 8.33
Total Return (29.60%) (4.14%)
Ratios to Average Net Assets    
Total income (loss) (31.58%) 3.98%
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.68% 0.54%
Net income (loss) (32.54%) 3.13%
[1] Annualized.