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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Per Share Operating Performance:    
Net asset value, beginning of period $ 9.26 $ 11.75
Total income (loss) 0.27 (1.68)
Net expenses (0.04) (0.05)
Net increase (decrease) in net asset value 0.23 (1.73)
Net asset value, end of period $ 9.49 $ 10.02
Total Return 2.48% (14.72%)
Ratios to Average Net Assets    
Total income (loss) 3.03% (20.02%)
Management fees [1] 0.75% 0.75%
Total expenses excluding management fees [1] 1.10% 0.57%
Expenses waived [1] (0.95%) (0.42%)
Net expenses excluding management fees [1] 0.15% 0.15%
Net income (loss) 2.58% (20.47%)
[1] Annualized.