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FINANCIAL HIGHLIGHTS (Details) - $ / shares
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Per Share Operating Performance:        
Net asset value, beginning of year $ 12.91 $ 20.91 $ 15.18  
Total income (loss) 8.14 (7.90) 5.90  
Total expenses (0.18) (0.10) (0.17)  
Net increase (decrease) in net asset value 7.96 (8.00) 5.73  
Net asset value, end of year $ 20.87 $ 12.91 $ 20.91 $ 15.18
Total Return 61.66% (38.26%) 37.75%  
Ratios to Average Net Assets        
Total income (loss) 53.29% 42.40% 31.50%  
Management fees 0.75% 0.75% 0.75%  
Total expenses excluding management fees 0.34%   0.38% 0.24%
Expenses waived (0.07%) (0.23%) (0.09%)  
Net expense excluding management fees 0.27% 0.15% 0.15%  
Net income (loss) 52.26% 41.50% 30.60%