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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Per Share Operating Performance:    
Net asset value, beginning of period $ 11.75 $ 9.72
Total income (loss) (1.68) 0.96
Net expenses (0.05) (0.04)
Net decrease in net asset value (1.73) 0.92
Net asset value, end of period $ 10.02 $ 10.64
Total Return (14.72%) 9.47%
Ratios to Average Net Assets    
Total income (loss) (20.02%) 10.53%
Management fees [1] 0.75% 0.75%
Total expenses excluding management fees [1] 0.57% 0.91%
Expenses waived [1] (0.42%) (0.76%)
Net expenses excluding management fees [1] 0.15% 0.15%
Net income (loss) (20.47%) 10.09%
[1] Annualized.