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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%