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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 | $ 20.39 | $ 17 | $ 26.59
Total income (loss) | 0.82 | 3.52 | (9.41)
Total expenses | (0.16) | (0.13) | (0.18)
Net increase (decrease) in net asset value | 0.66 | 3.39 | (9.59)
Net asset value, end of year | $ 21.05 | $ 20.39 | $ 17
Total Return | 3.24% | 19.94% | (36.07%)
Ratios to Average Net Assets |  |  |
Total income (loss) | 2.46% | 27.09% | (37.86%)
Management fees | 0.60% | 0.60% | 0.60%
Expenses excluding management fees | 0.26% | 0.12% | 0.20%
Net income (loss) | 1.60% | 26.36% | (38.65%)

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