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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, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Per Share Operating Performance:        
Net asset value, beginning of year $ 9.61 $ 12.43 $ 9.20 $ 16.91
Total income (loss) 3.18 (2.13) 3.61 (6.57)
Total expenses (0.04) (0.05) (0.06) (0.09)
Net increase (decrease) in net asset value 3.14 (2.18) 3.55 (6.66)
Net asset value, end of period $ 12.75 $ 10.25 $ 12.75 $ 10.25
Total Return 32.67% (17.54%) 38.59% (39.38%)
Ratios to Average Net Assets        
Total income (loss) 29.45% (18.27%) 34.79% (47.61%)
Management fees [1] 0.60% 0.60% 0.60% 0.60%
Total expenses excluding management fees [1] 0.80% 0.96% 0.67% 0.84%
Net income (loss) 29.10% (18.66%) 34.16% (48.33%)
[1] #    Annualized.