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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Per Share Operating Performance:        
Net asset value, beginning of period $ 12.43 $ 23.42 $ 16.91 $ 24.35
Total income (loss) (2.13) (3.52) (6.57) (4.36)
Total expenses (0.05) (0.07) (0.09) (0.16)
Net increase (decrease) in net asset value (2.18) (3.59) (6.66) (4.52)
Net asset value, end of period $ 10.25 $ 19.83 $ 10.25 $ 19.83
Total Return (17.54%) (15.33%) (39.38%) (18.56%)
Ratios to Average Net Assets        
Total income (loss) (18.27%) (14.40%) (47.61%) (13.85%)
Management fees [1] 0.60% 0.60% 0.60% 0.60%
Expenses excluding management fees [1] 0.96% 0.39% 0.84% 0.78%
Net income (loss) (18.66%) (14.74%) (48.33%) (14.53%)
[1] Annualized.