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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.

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