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Per Unit Performance Data and Other Supplemental Financial Data (Detail) (USD $)
 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Nov. 18, 2009
Per Unit Operating Performance:      
Net Asset Value Per Unit, Beginning $ 21.21 $ 35.02 $ 50.00
Total income (loss) $ (3.74) $ (2.29)  
Total expenses $ (0.09) $ (0.16)  
Net decrease in net asset value $ (3.83) $ (2.45)  
Net Asset Value Per Unit, Ending $ 17.38 $ 32.57 $ 50.00
Total Return (18.06%) (7.00%)  
Ratios to Average Net Assets      
Total income (loss) (15.50%) (6.81%)  
Management fees 0.75% [1] 0.75% [1]  
Total expenses excluding management fees 0.44% [1] 0.64% [1]  
Expenses waived (0.20%) [1] (0.43%) [1]  
Net expenses excluding management fees 0.24% [1] 0.21% [1]  
Net income (loss) (15.99%) (7.28%)  
[1] Annualized