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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Per Share Operating Performance:    
Net asset value, beginning of period $ 26.59 $ 42.83
Total income (loss) 0.36 4.55
Total expenses (0.11) (0.21)
Net increase (decrease) in net asset value 0.25 4.34
Net asset value, end of period $ 26.84 $ 47.17
Total Return 0.94% 10.13%
Ratios to Average Net Assets    
Total income (loss) 5.13% 9.78%
Expenses excluding management fees [1] 0.22% 0.37%
Management fees [1] 0.60% 0.60%
Net income (loss) 4.72% 9.30%
[1] Annualized