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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Per Share Operating Performance:    
Net asset value, beginning of year $ 20.39 $ 17
Total income (loss) (3.52) 2.4
Net expenses (0.08) (0.06)
Net increase (decrease) in net asset value (3.6) 2.34
Net asset value, end of year $ 16.79 $ 19.34
Total Return (17.66%) 13.76%
Ratios to Average Net Assets    
Total income (loss) (18.40%) 20.77%
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.27% 0.13%
Net income (loss) (18.83%) 20.40%
[1] Annualized.