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FINANCIAL HIGHLIGHTS (Tables) | 12 Months Ended
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Dec. 31, 2018
Financial Highlights [Abstract] |
Per Unit Performance Data and Other Supplemental Financial Data | The following table presents per share performance data and other supplemental financial data for the years ended December 31, 2018, 2017 and 2016 for the shareholders. This information has been derived from information presented in the financial statements.
|  | Year ended December 31, 2018 |  |  | Year ended December 31, 2017 |  |  | Year ended December 31, 2016 |
Per Share Operating Performance: |  |  |  |  |  |  |  |  |  |  |  |
Net asset value, beginning of year |  | $ | 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 year |  | $ | 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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