Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) (USD $)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2009
Dec. 06, 2007
Per Unit Operating Performance:          
Net Asset Value Per Share, Beginning $ 39.81 $ 43.46 $ 42.91 $ 42.91 $ 50.00
Total income (loss) $ 3.43 $ (3.32) $ 0.88    
Net expenses $ (0.41) $ (0.33) $ (0.33)    
Net increase (decrease) in net asset value $ 3.02 $ (3.65) $ 0.55    
Net Asset Value Per Share, Ending $ 42.83 $ 39.81 $ 43.46 $ 42.91 $ 50.00
Total Return 7.59% (8.40%) 1.28%    
Ratios to Average Net Assets          
Total income (loss) 7.54% (6.94%) 2.08%    
Management fees 0.60% 0.60% 0.60%    
Expenses excluding management fees 0.39% 0.18% 0.17%    
Net income (loss) 6.55% (7.72%) 1.31%