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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Per Share Operating Performance:    
Net asset value, beginning of period $ 26.59 $ 42.83
Total income (loss) (5.99) (0.72)
Total expenses (0.15) (0.31)
Net increase (decrease) in net asset value (6.14) (1.03)
Net asset value, end of period $ 20.45 $ 41.80
Total Return (23.09%) (2.40%)
Ratios to Average Net Assets    
Total income (loss) (20.93%) (1.31%)
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.20% 0.33%
Net income (loss) (21.53%) (2.00%)
[1] Annualized