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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) (USD $)
 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Dec. 06, 2007
Per Unit Operating Performance:      
Net asset value, beginning of period $ 39.81 $ 43.46 $ 50.00
Total income (loss) $ 0.46 $ (5.40)  
Net expenses $ (0.20) $ (0.16)  
Net increase (decrease) in net asset value $ 0.26 $ (5.56)  
Net asset value, end of period $ 40.07 $ 37.90 $ 50.00
Total Return 0.65% (12.79%)  
Ratios to Average Net Assets      
Total income (loss) 1.27% (10.50%)  
Expenses excluding management fees 0.39% [1] 0.15% [1]  
Management fees 0.60% [1] 0.60% [1]  
Net income (loss) 0.78% (10.87%)  
[1] Annualized