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FINANCIAL HIGHLIGHTS (Details) - $ / shares
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Per Share Operating Performance:      
Net asset value, beginning of period $ 22.95 $ 17.82 $ 21.05
Total income (loss) (5.59) 5.32 (3.04)
Total expenses (0.13) (0.19) (0.19)
Net increase (decrease) in net asset value (5.72) 5.13 (3.23)
Net asset value, end of period $ 17.23 $ 22.95 $ 17.82
Total Return (24.92%) 28.79% (15.34%)
Ratios to Average Net Assets      
Total income (loss) 61.24% 26.56% (6.16%)
Management fees 0.60% 0.60% 0.60%
Expenses excluding management fees 0.28% 0.28% 0.22%
Net income (loss) 60.36% 25.68% (6.98%)