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Financial Highlights (Tables) | 12 Months Ended
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Dec. 31, 2014
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, 2014, 2013 and 2012. This information has been derived from information presented in the financial statements.
|  | Year ended December 31, 2014 |  |  | Year ended December 31, 2013 |  |  | Year ended December 31, 2012 |
Per Share Operating Performance: |  |  |  |  |  |  |  |  |  |  |  |
Net asset value, beginning of year |  | $ | 42.83 |  |  | $ | 39.81 |  |  | $ | 43.46 |
Total income (loss) |  |  | (15.85) |  |  |  | 3.43 |  |  |  | (3.32) |
Net expenses |  |  | (0.39) |  |  |  | (0.41) |  |  |  | (0.33) |
Net increase (decrease) in net asset value |  |  | (16.24) |  |  |  | 3.02 |  |  |  | (3.65) |
Net asset value, end of year |  | $ | 26.59 |  |  | $ | 42.83 |  |  | $ | 39.81 |
Total Return |  |  | (37.92) | % |  |  | 7.59 | % |  |  | (8.40) | %
Ratios to Average Net Assets |  |  |  |  |  |  |  |  |  |  |  |
Total income (loss) |  |  | (34.74) | % |  |  | 7.54 | % |  |  | (6.94) | %
Management fees |  |  | 0.60 | % |  |  | 0.60 | % |  |  | 0.60 | %
Expenses excluding management fees |  |  | 0.33 | % |  |  | 0.39 | % |  |  | 0.18 | %
Net income (loss) |  |  | (35.66) | % |  |  | 6.55 | % |  |  | (7.72) | %

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