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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares | 6 Months Ended
--- | ---
Jun. 30, 2018 | Jun. 30, 2017
Per Share Operating Performance: |  |  |
Net asset value, beginning of period |  | $ 21.05 | $ 20.39
Total income (loss) |  | 4.41 | (3.52)
Total expenses |  | (0.09) | (0.08)
Net increase (decrease) in net asset value |  | 4.32 | (3.60)
Net asset value, end of period |  | $ 25.37 | $ 16.79
Total Return |  | 20.52% | (17.66%)
Ratios to Average Net Assets |  |  |
Total income (loss) |  | 19.19% | (18.40%)
Management fees | [1] | 0.60% | 0.60%
Expenses excluding management fees | [1] | 0.21% | 0.27%
Net income (loss) |  | 18.78% | (18.83%)
[1] | Annualized.

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