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FINANCIAL HIGHLIGHTS (Details) - $ / shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Per Share Operating Performance:      
Net asset value, beginning of year $ 18.18 $ 15.70 $ 12.22
Total income (loss) (2.82) 2.61 3.60
Net expenses (0.18) (0.13) (0.12)
Net increase (decrease) in net asset value (3.00) 2.48 3.48
Net asset value, end of year $ 15.18 $ 18.18 $ 15.70
Total Return (16.50%) 15.80% 28.48%
Ratios to Average Net Assets      
Total income (loss) (14.88%) 16.22% 34.23%
Management fees 0.75% 0.75% 0.75%
Total expenses excluding management fees 0.23% 0.48% 0.32%
Expenses waived (0.08%) (0.33%) (0.17%)
Net expenses excluding management fees 0.15% 0.15% 0.15%
Net income (loss) (15.78%) 15.32% 33.33%