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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Per Share Operating Performance:    
Net Asset Value, beginning of period $ 11.75 $ 9.72
Total income (loss) (1.67) 0.48
Net expenses (0.07) (0.06)
Net increase (decrease) in net asset value (1.74) 0.42
Net Asset Value, end of period $ 10.01 $ 10.14
Total Return (14.81%) 4.32%
Ratios to Average Net Assets    
Total income (loss) (21.53%) 4.93%
Management fees [1] 0.75% 0.75%
Total expenses excluding management fees [1] 0.68% 0.93%
Expenses waived [1] (0.53%) (0.78%)
Net expenses excluding management fees [1] 0.15% 0.15%
Net income (loss) (22.20%) 4.26%
[1] Annualized.