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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 year $ 20.39 $ 17.00
Total income (loss) (1.97) 1.84
Total expenses (0.11) (0.09)
Net increase (decrease) in net asset value (2.08) 1.75
Net asset value, end of year $ 18.31 $ 18.75
Total Return (10.20%) 10.29%
Ratios to Average Net Assets    
Total income (loss) (9.42%) 17.46%
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.23% 0.12%
Net income (loss) (10.04%) 16.91%
[1] Annualized.