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FINANCIAL HIGHLIGHTS (Details) - $ / shares
 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Per Share Operating Performance:    
Net asset value, beginning of year $ 15.18 $ 18.18
Total income (loss) 4.03 4.12
Net expenses (0.08) (0.09)
Net increase (decrease) in net asset value 3.95 4.03
Net asset value, end of year $ 19.13 $ 22.21
Total Return 26.02% 22.17%
Ratios to Average Net Assets    
Total income (loss) 22.16% 20.05%
Management fees 0.75% 0.75%
Total expenses excluding management fees 0.17% 0.31%
Expenses waived (0.02%) (0.16%)
Net expenses excluding management fees 0.15% 0.15%
Net income (loss) 21.72% 19.61%