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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 $ 10.26 $ 9.26 $ 11.75
Total income (loss) (1.74) 1.09 (2.40)
Net expenses (0.09) (0.09) (0.09)
Net increase (decrease) in net asset value (1.83) 1.00 (2.49)
Net asset value, end of year $ 8.43 $ 10.26 $ 9.26
Total Return (17.84%) 10.80% (21.19%)
Ratios to Average Net Assets      
Total income (loss) (17.87%) 9.73% (29.05%)
Management fees 0.75% 0.75% 0.75%
Total expenses excluding management fees 1.86% 1.08% 0.74%
Expenses waived (1.71%) (0.93%) (0.59%)
Net expenses excluding management fees 0.15% 0.15% 0.15%
Net income (loss) (18.77%) 8.83% (29.95%)