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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Per Share Operating Performance: |  |  |
Net asset value, beginning of period | $ 21.05 | $ 20.39 | $ 17.00
Total income (loss) | (3.04) | 0.82 | 3.52
Total expenses | (0.19) | (0.16) | (0.13)
Net increase (decrease) in net asset value | (3.23) | 0.66 | 3.39
Net asset value, end of period | $ 17.82 | $ 21.05 | $ 20.39
Total Return | (15.34%) | 3.24% | 19.94%
Ratios to Average Net Assets |  |  |
Total income (loss) | (6.16%) | 2.46% | 27.09%
Management fees | 0.60% | 0.60% | 0.60%
Expenses excluding management fees | 0.22% | 0.26% | 0.12%
Net income (loss) | (6.98%) | 1.60% | 26.36%

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