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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Per Share Operating Performance:    
Net asset value, beginning of period $ 9.26 $ 11.75
Total income (loss) 0.25 (1.67)
Net expenses (0.06) (0.07)
Net increase (decrease) in net asset value 0.19 (1.74)
Net asset value, end of period $ 9.45 $ 10.01
Total Return 2.05% (14.81%)
Ratios to Average Net Assets    
Total income (loss) 2.89% (21.53%)
Management fees [1] 0.75% 0.75%
Total expenses excluding management fees [1] 1.06% 0.68%
Expenses waived [1] (0.91%) (0.53%)
Net expenses excluding management fees [1] 0.15% 0.15%
Net income (loss) 2.22% (22.20%)
[1] Annualized.