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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
 [1]
Per Share Operating Performance:      
Net asset value, beginning of year $ 16.91 $ 24.35 [1] $ 23.34
Total income (loss) (7.55) (7.16) 1.30
Total expenses (0.16) (0.28) (0.29)
Net increase (decrease) in net asset value (7.71) (7.44) 1.01
Net asset value, end of year $ 9.20 $ 16.91 $ 24.35
Total Return (45.59%) (30.55%) 4.33%
Ratios to Average Net Assets      
Total income (loss) (49.07%) (24.25%) 30.51%
Management fees 0.60% 0.60% 0.60%
Expenses excluding management fees 0.75% 0.73% 0.68%
Net income (loss) (50.42%) (25.58%) 29.23%
[1] On January 4, 2018, there was a 1-for-4 reverse share split. Historical shares outstanding, net asset value per share and market value per share have been adjusted for the period shown to reflect the 1-for-4 reverse share split on a retroactive basis.