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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Per Share Operating Performance:    
Net asset value, beginning of year $ 11.75 $ 9.72
Total income (loss) (1.08) (0.96)
Net expenses (0.02) (0.02)
Net increase (decrease) in net asset value (1.1) (0.98)
Net asset value, end of year $ 10.65 $ 8.74
Total Return (9.36%) (10.08%)
Ratios to Average Net Assets    
Total income (loss) (13.70%) (11.06%)
Management fees [1] 0.75% 0.75%
Total expenses excluding management fees [1] 0.59% 0.93%
Expenses waived [1] (0.44%) (0.78%)
Net expenses excluding management fees [1] 0.15% 0.15%
Net income (loss) (13.92%) (11.28%)
[1] Annualized.