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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 period $ 17.82 $ 21.05
Total income (loss) 4.08 4.41
Total expenses (0.10) (0.09)
Net increase (decrease) in net asset value 3.98 4.32
Net asset value, end of period $ 21.80 $ 25.37
Total Return 22.33% 20.52%
Ratios to Average Net Assets    
Total income (loss) 20.09% 19.19%
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.31% 0.21%
Net income (loss) 19.64% 18.78%
[1] Annualized.