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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017
Per Share Operating Performance: |  |  |  |
Net asset value, beginning of period | [1] | $ 23.34 | $ 37.18 |
Total income (loss) |  | 0.50 | (9.85) | [1]
Total expenses |  | (0.13) | (0.20) | [1]
Net increase (decrease) in net asset value |  | 0.37 | (10.05) | [1]
Net asset value, end of period |  | $ 23.71 | $ 27.13 | [1]
Total Return |  | 1.59% | (27.02%) |
Ratios to Average Net Assets |  |  |  |
Total income (loss) |  | 8.31% | (29.11%) |
Management fees | [2] | 0.60% | 0.60% |
Expenses excluding management fees | [2] | 0.71% | 0.73% |
Net income (loss) |  | 7.66% | (29.77%) |
[1] | On January 4, 2018, there was a 1-for-4 reverse share split. Historical shares outstanding, net asset value per share and market value per share have been adjusted to reflect the 1-for-4 reverse share split on a retroactive basis.
[2] | Annualized.

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