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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2013 | Mar. 31, 2012 | Dec. 06, 2007
Per Unit Operating Performance: |  |  |  |  |
Net asset value, beginning of period | $ 39.81 |  | $ 43.46 |  | $ 50.00
Total income (loss) | $ 1.32 |  | $ 2.43 |  |
Net expenses | $ (0.10) |  | $ (0.08) |  |
Net increase in net asset value | $ 1.22 |  | $ 2.35 |  |
Net asset value, end of period | $ 41.03 |  | $ 45.81 |  | $ 50.00
Total Return | 3.06% |  | 5.41% |  |
Ratios to Average Net Assets |  |  |  |  |
Total income (loss) | 3.36% |  | 5.52% |  |
Expenses excluding management fees | 0.36% | [1] | 0.14% | [1] |
Management fees | 0.60% | [1] | 0.60% | [1] |
Net income (loss) | 3.12% |  | 5.34% |  |
[1] | * Annualized

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