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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 | 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 |  | $ | 9.26 |  |  | $ | 11.75 |  |  | $ | 9.72 |
Total income (loss) |  |  | 1.09 |  |  |  | (2.40 | ) |  |  | 2.12 |
Net expenses |  |  | (0.09 | ) |  |  | (0.09 | ) |  |  | (0.09 | )
Net increase (decrease) in net asset value |  |  | 1.00 |  |  |  | (2.49 | ) |  |  | 2.03 |
Net asset value, end of year |  | $ | 10.26 |  |  | $ | 9.26 |  |  | $ | 11.75 |
Total Return |  |  | 10.80 | % |  |  | (21.19 | )% |  |  | 20.88 | %
Ratios to Average Net Assets |  |  |  |  |  |  |  |  |  |  |  |
Total income (loss) |  |  | 9.73 | % |  |  | (29.05 | )% |  |  | 21.94 | %
Management fees |  |  | 0.75 | % |  |  | 0.75 | % |  |  | 0.75 | %
Total expenses excluding management fees* |  |  | 1.08 | % |  |  | 0.74 | % |  |  | 0.90 | %
Expenses waived |  |  | (0.93 | )% |  |  | (0.59 | )% |  |  | (0.75 | )%
Net expenses excluding management fees |  |  | 0.15 | % |  |  | 0.15 | % |  |  | 0.15 | %
Net income (loss) |  |  | 8.83 | % |  |  | (29.95 | )% |  |  | 21.04 | %

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