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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Per Share Operating Performance: |  |  |
Net asset value, beginning of year | $ 11.75 | $ 9.72 | $ 13.69
Total income (loss) | (2.4) | 2.12 | (3.86)
Net expenses | (0.09) | (0.09) | (0.11)
Net increase (decrease) in net asset value | (2.49) | 2.03 | (3.97)
Net asset value, end of year | $ 9.26 | $ 11.75 | $ 9.72
Total Return | (21.19%) | 20.88% | (29.00%)
Ratios to Average Net Assets |  |  |
Total income (loss) | (29.05%) | 21.94% | (27.80%)
Management fees | 0.75% | 0.75% | 0.75%
Total expenses excluding management fees | 0.74% | 0.90% | 0.71%
Expenses waived | (0.59%) | (0.75%) | (0.56%)
Net expenses excluding management fees | 0.15% | 0.15% | 0.15%
Net income (loss) | (29.95%) | 21.04% | (28.70%)

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