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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 $ 21.05 $ 20.39
Total income (loss) 4.41 (3.52)
Total expenses (0.09) (0.08)
Net increase (decrease) in net asset value 4.32 (3.60)
Net asset value, end of period $ 25.37 $ 16.79
Total Return 20.52% (17.66%)
Ratios to Average Net Assets    
Total income (loss) 19.19% (18.40%)
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.21% 0.27%
Net income (loss) 18.78% (18.83%)
[1] Annualized.