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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%)