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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Per Share Operating Performance: |  |  |
Net asset value, beginning of period |  | $ 21.05 | $ 20.39
Total income (loss) |  | 1.64 | (1.83)
Total expenses |  | (0.04) | (0.04)
Net increase (decrease) in net asset value |  | 1.6 | (1.87)
Net asset value, end of period |  | $ 22.65 | $ 18.52
Total Return |  | 7.60% | (9.17%)
Ratios to Average Net Assets |  |  |
Total income (loss) |  | 7.71% | (9.31%)
Management fees | [1] | 0.60% | 0.60%
Expenses excluding management fees | [1] | 0.21% | 0.29%
Net income (loss) |  | 7.51% | (9.53%)
[1] | Annualized.

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