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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Per Share Operating Performance:    
Net asset value, beginning of period $ 17.82 $ 21.05
Total income (loss) 4.75 1.64
Total expenses (0.05) (0.04)
Net increase (decrease) in net asset value 4.70 1.60
Net asset value, end of period $ 22.52 $ 22.65
Total Return 26.37% 7.60%
Ratios to Average Net Assets    
Total income (loss) 22.55% 7.71%
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.31% 0.21%
Net income (loss) 22.32% 7.51%
[1] Annualized.