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Per Unit Performance Data and Other Supplemental Financial Data (Detail) (USD $) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Nov. 18, 2009
Per Unit Operating Performance: |  |  |  |  |
Net asset value, beginning of period | $ 21.21 |  | $ 35.02 |  | $ 50.00
Total income (loss) | $ (2.40) |  | $ (6.85) |  |
Total expenses | $ (0.13) |  | $ (0.23) |  |
Net decrease in net asset value | $ (2.53) |  | $ (7.08) |  |
Net asset value, end of period | $ 18.68 |  | $ 27.94 |  | $ 50.00
Total Return | (11.93%) |  | (20.22%) |  |
Ratios to Average Net Assets |  |  |  |  |
Total income (loss) | (2.93%) |  | (21.56%) |  |
Management fees* | 0.75% | [1] | 0.75% | [1] |
Total expenses excluding management fees* | 0.39% | [1] | 0.65% | [1] |
Expenses waived* | (0.16%) | [1] | (0.44%) | [1] |
Net expenses excluding management fees* | 0.23% | [1] | 0.21% | [1] |
Net income (loss) | (3.66%) |  | (22.28%) |  |
[1] | Annualized.

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