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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) (USD $) | 6 Months Ended |
--- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Dec. 06, 2007
Per Unit Operating Performance: |  |  |  |  |
Net asset value, beginning of period | $ 39.81 |  | $ 43.46 |  | $ 50.00
Total income (loss) | $ 0.46 |  | $ (5.40) |  |
Net expenses | $ (0.20) |  | $ (0.16) |  |
Net increase (decrease) in net asset value | $ 0.26 |  | $ (5.56) |  |
Net asset value, end of period | $ 40.07 |  | $ 37.90 |  | $ 50.00
Total Return | 0.65% |  | (12.79%) |  |
Ratios to Average Net Assets |  |  |  |  |
Total income (loss) | 1.27% |  | (10.50%) |  |
Expenses excluding management fees | 0.39% | [1] | 0.15% | [1] |
Management fees | 0.60% | [1] | 0.60% | [1] |
Net income (loss) | 0.78% |  | (10.87%) |  |
[1] | Annualized

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