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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 $ 21.05 $ 20.39
Total income (loss) 6.23 (1.97)
Total expenses (0.14) (0.11)
Net increase (decrease) in net asset value 6.09 (2.08)
Net asset value, end of period $ 27.14 $ 18.31
Total Return 28.93% (10.20%)
Ratios to Average Net Assets    
Total income (loss) 26.11% (9.42%)
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.21% 0.23%
Net income (loss) 25.50% (10.04%)
[1] Annualized.