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FINANCIAL HIGHLIGHTS - Per Unit Performance Data and Other Supplemental Financial Data (Details) - $ / shares | 6 Months Ended
--- | ---
Jun. 30, 2019 | Jun. 30, 2018
Per Share Operating Performance: |  |  |
Net asset value, beginning of period |  | $ 24.35 | $ 23.34
Total income (loss) |  | (4.36) | 0.50
Total expenses |  | (0.16) | (0.13)
Net increase (decrease) in net asset value |  | (4.52) | 0.37
Net asset value, end of period |  | $ 19.83 | $ 23.71
Total Return |  | (18.56%) | 1.59%
Ratios to Average Net Assets |  |  |
Total income (loss) |  | (13.85%) | 8.31%
Management fees | [1] | 0.60% | 0.60%
Expenses excluding management fees | [1] | 0.78% | 0.71%
Net income (loss) |  | (14.53%) | 7.66%
[1] | Annualized.

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