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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Per Share Operating Performance:    
Net asset value, beginning of period $ 17 $ 26.59
Total income (loss) (0.62) (2.58)
Total expenses (0.03) (0.05)
Net increase (decrease) in net asset value (0.65) (2.63)
Net asset value, end of period $ 16.35 $ 23.96
Total Return (3.82%) (9.89%)
Ratios to Average Net Assets    
Total income (loss) 1.73% (8.12%)
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.17% 0.21%
Net income (loss) 1.54% (8.32%)
[1] Annualized.