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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Per Share Operating Performance: |  |  |
Net asset value, beginning of year | $ 9.72 | $ 13.69 | $ 18.32
Total income (loss) | 2.12 | (3.86) | (4.33)
Net expenses | (0.09) | (0.11) | (0.30)
Net increase (decrease) in net asset value | 2.03 | (3.97) | (4.63)
Net asset value, end of year | $ 11.75 | $ 9.72 | $ 13.69
Total Return | 20.88% | (29.00%) | (25.27%)
Ratios to Average Net Assets |  |  |
Total income (loss) | 21.94% | (27.80%) | (19.76%)
Management fees | 0.75% | 0.75% | 0.75%
Total expenses excluding management fees | 0.90% | 0.71% | 0.85%
Expenses waived | (0.75%) | (0.56%) | (0.67%)
Net expenses excluding management fees | 0.15% | 0.15% | 0.18%
Net income (loss) | 21.04% | (28.70%) | (20.70%)

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