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FINANCIAL HIGHLIGHTS - Per Unit Performance Data and Other Supplemental Financial Data (Details) - $ / shares
 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Per Share Operating Performance:    
Net asset value, beginning of period $ 24.35 $ 23.34
Total income (loss) (4.36) 0.50
Total expenses (0.16) (0.13)
Net increase (decrease) in net asset value (4.52) 0.37
Net asset value, end of period $ 19.83 $ 23.71
Total Return (18.56%) 1.59%
Ratios to Average Net Assets    
Total income (loss) (13.85%) 8.31%
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.78% 0.71%
Net income (loss) (14.53%) 7.66%
[1] Annualized.