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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) (USD $)
 3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Dec. 06, 2007
Per Unit Operating Performance:      
Net Asset Value Per Share, Beginning $ 42.83 $ 39.81 $ 50.00
Total income $ 1.38 $ 1.32  
Total expenses $ (0.11) $ (0.10)  
Net increase in net asset value $ 1.27 $ 1.22  
Net Asset Value Per Share, Ending $ 44.10 $ 41.03 $ 50.00
Total Return 2.97% 3.06%  
Ratios to Average Net Assets      
Total income 2.80% 3.36%  
Management fees 0.60% [1] 0.60% [1]  
Expenses excluding management fees 0.41% [1] 0.36% [1]  
Net income 2.55% 3.12%  
[1] Annualized