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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.

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