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FINANCIAL HIGHLIGHTS (Details) - $ / shares
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Per Share Operating Performance:      
Net asset value, beginning of year $ 17.82 $ 21.05 $ 20.39
Total income (loss) 5.32 (3.04) 0.82
Total expenses (0.19) (0.19) (0.16)
Net increase (decrease) in net asset value 5.13 (3.23) 0.66
Net asset value, end of year $ 22.95 $ 17.82 $ 21.05
Total Return 28.79% (15.34%) 3.24%
Ratios to Average Net Assets      
Total income (loss) 26.56% (6.16%) 2.46%
Management fees 0.60% 0.60% 0.60%
Expenses excluding management fees 0.28% 0.22% 0.26%
Net income (loss) 25.68% (6.98%) 1.60%