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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 year |  | $ 20.39 | $ 17.00
Total income (loss) |  | (1.97) | 1.84
Total expenses |  | (0.11) | (0.09)
Net increase (decrease) in net asset value |  | (2.08) | 1.75
Net asset value, end of year |  | $ 18.31 | $ 18.75
Total Return |  | (10.20%) | 10.29%
Ratios to Average Net Assets |  |  |
Total income (loss) |  | (9.42%) | 17.46%
Management fees | [1] | 0.60% | 0.60%
Expenses excluding management fees | [1] | 0.23% | 0.12%
Net income (loss) |  | (10.04%) | 16.91%
[1] | Annualized.

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