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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 |  | $ 9.26 | $ 11.75
Total income (loss) |  | 0.27 | (1.68)
Net expenses |  | (0.04) | (0.05)
Net increase (decrease) in net asset value |  | 0.23 | (1.73)
Net asset value, end of period |  | $ 9.49 | $ 10.02
Total Return |  | 2.48% | (14.72%)
Ratios to Average Net Assets |  |  |
Total income (loss) |  | 3.03% | (20.02%)
Management fees | [1] | 0.75% | 0.75%
Total expenses excluding management fees | [1] | 1.10% | 0.57%
Expenses waived | [1] | (0.95%) | (0.42%)
Net expenses excluding management fees | [1] | 0.15% | 0.15%
Net income (loss) |  | 2.58% | (20.47%)
[1] | Annualized.

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