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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares | 9 Months Ended
--- | ---
Sep. 30, 2015 | Sep. 30, 2014
Per Share Operating Performance: |  |  |
Net asset value, beginning of period |  | $ 26.59 | $ 42.83
Total income (loss) |  | (5.99) | (0.72)
Total expenses |  | (0.15) | (0.31)
Net increase (decrease) in net asset value |  | (6.14) | (1.03)
Net asset value, end of period |  | $ 20.45 | $ 41.80
Total Return |  | (23.09%) | (2.40%)
Ratios to Average Net Assets |  |  |
Total income (loss) |  | (20.93%) | (1.31%)
Management fees | [1] | 0.60% | 0.60%
Expenses excluding management fees | [1] | 0.20% | 0.33%
Net income (loss) |  | (21.53%) | (2.00%)
[1] | Annualized

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