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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) (USD $)
 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Dec. 06, 2007
Per Unit Operating Performance:      
Net Asset Value Per Share, Beginning $ 42.83 $ 39.81 $ 50.00
Total income $ 4.55 $ 0.46  
Total expenses $ (0.21) $ (0.20)  
Net increase in net asset value $ 4.34 $ 0.26  
Net Asset Value Per Share, Ending $ 47.17 $ 40.07 $ 50.00
Total Return 10.13% 0.65%  
Ratios to Average Net Assets      
Total income 9.78% 1.27%  
Management fees 0.60% [1] 0.60% [1]  
Expenses excluding management fees 0.37% [1] 0.39% [1]  
Net income 9.30% 0.78%  
[1] Annualized