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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares | 6 Months Ended
--- | ---
Jun. 30, 2015 | Jun. 30, 2014
Per Share Operating Performance: |  |  |
Net asset value, beginning of period |  | $ 26.59 | $ 42.83
Total income (loss) |  | 0.36 | 4.55
Total expenses |  | (0.11) | (0.21)
Net increase (decrease) in net asset value |  | 0.25 | 4.34
Net asset value, end of period |  | $ 26.84 | $ 47.17
Total Return |  | 0.94% | 10.13%
Ratios to Average Net Assets |  |  |
Total income (loss) |  | 5.13% | 9.78%
Expenses excluding management fees | [1] | 0.22% | 0.37%
Management fees | [1] | 0.60% | 0.60%
Net income (loss) |  | 4.72% | 9.30%
[1] | Annualized

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