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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Per Share Operating Performance: |  |  |
Net asset value, beginning of period | $ 26.59 | $ 42.83 | $ 39.81
Total income (loss) | (9.41) | (15.85) | 3.43
Net expenses | (0.18) | (0.39) | (0.41)
Net increase (decrease) in net asset value | (9.59) | (16.24) | 3.02
Net asset value, end of period | $ 17 | $ 26.59 | $ 42.83
Total Return | (36.07%) | (37.92%) | 7.59%
Ratios to Average Net Assets |  |  |
Total income (loss) | (37.86%) | (34.74%) | 7.54%
Management fees | 0.60% | 0.60% | 0.60%
Expenses excluding management fees | 0.20% | 0.33% | 0.39%
Net income (loss) | (38.65%) | (35.66%) | 6.55%

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