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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Per Share Operating Performance: |  |  |  |  |  |  |
Net asset value, beginning of year | [1],[3] | $ 23.34 | [2] | $ 37.18 |  | $ 34.76 |
Total income (loss) |  | 1.30 |  | (13.49) | [1] | 2.77 | [1]
Total expenses |  | (0.29) |  | (0.35) | [1] | (0.35) | [1]
Net increase (decrease) in net asset value |  | 1.01 |  | (13.84) | [1] | 2.42 | [1]
Net asset value, end of year |  | $ 24.35 |  | $ 23.34 | [1],[2],[3] | $ 37.18 | [1],[3]
Total Return |  | 4.33% |  | (37.22%) |  | 6.90% |
Ratios to Average Net Assets |  |  |  |  |  |  |
Total income (loss) |  | 30.51% |  | (37.75%) |  | 19.27% |
Management fees |  | 0.60% |  | 0.60% |  | 0.60% |
Expenses excluding management fees | [1] | 0.68% |  | 0.70% |  | 0.52% |
Net income (loss) |  | 29.23% |  | (39.05%) |  | 18.15% |
[1] | On January 4, 2018, there was a 1-for-4 reverse share split. Historical shares outstanding, net asset value per share and market value per share have been adjusted for the period shown to reflect the 1-for-4 reverse share split on a retroactive basis.
[2] | On January 4, 2018, there was a 1-for-4 reverse share split. Historical shares outstanding, net asset value per share and market value per share have been adjusted to reflect the 1-for-4 reverse share split on a retroactive basis.
[3] | On January 4, 2018, there was a 1-for-4 reverse share split. The Statement of Changes in Partners’ Capital have been adjusted for the period shown to reflect the 1-for-4 reverse share split on a retroactive basis.

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