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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Per Share Operating Performance:        
Net asset value, beginning of year $ 12.75 $ 10.25 $ 9.20 $ 16.91
Total income (loss) 7.58 1.14 11.19 (5.43)
Total expenses (0.04) (0.04) (0.10) (0.13)
Net increase (decrease) in net asset value 7.54 1.10 11.09 (5.56)
Net asset value, end of period $ 20.29 $ 11.35 $ 20.29 $ 11.35
Total Return 59.14% 10.73% 120.54% (32.88%)
Ratios to Average Net Assets        
Total income (loss) 47.64% 14.92% 80.90% (36.50%)
Management fees [1] 0.60% 0.60% 0.60% 0.60%
Total expenses excluding management fees [1] 0.46% 0.73% 0.60% 0.81%
Net income (loss) 47.37% 14.59% 80.00% (37.56%)
[1] #    Annualized.