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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 | [1] | $ 23.34 | $ 37.18 |
Total income (loss) |  | (0.73) | (6.80) | [1]
Total expenses |  | (0.07) | (0.10) | [1]
Net increase (decrease) in net asset value |  | (0.80) | (6.90) | [1]
Net asset value, end of period |  | $ 22.54 | $ 30.28 | [1]
Total Return |  | (3.43%) | (18.51%) |
Ratios to Average Net Assets |  |  |  |
Total income (loss) |  | 2.85% | (18.65%) |
Management fees | [2] | 0.60% | 0.60% |
Expenses excluding management fees | [2] | 0.70% | 0.73% |
Net income (loss) |  | 2.53% | (18.97%) |
[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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