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Per Unit Performance Data and Other Supplemental Financial Data (Detail) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 06, 2007
Per Unit Operating Performance: |  |  |  |
Net Asset Value Per Unit, Beginning | $ 43.46 | $ 42.91 | $ 40.37 | $ 50.00
Total income (loss) | $ (3.32) | $ 0.88 | $ 2.87 |
Net expenses | $ (0.33) | $ (0.33) | $ (0.33) |
Net increase (decrease) in net asset value | $ (3.65) | $ 0.55 | $ 2.54 |
Net Asset Value Per Unit, Ending | $ 39.81 | $ 43.46 | $ 42.91 | $ 50.00
Total Return | (8.40%) | 1.28% | 6.29% |
Ratios to Average Net Assets |  |  |  |
Total income (loss) | (6.94%) | 2.08% | 7.93% |
Management fees | 0.60% | 0.60% | 0.60% |
Expenses excluding management fees | 0.18% | 0.17% | 0.24% |
Net income (loss) | (7.72%) | 1.31% | 7.09% |

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