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FINANCIAL HIGHLIGHTS (Details) - $ / shares
 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Per Share Operating Performance:    
Net asset value, beginning of period $ 18.18 $ 15.70
Total income (loss) 4.12 (2.67)
Net expenses (0.09) (0.06)
Net increase (decrease) in net asset value 4.03 (2.73)
Net asset value, end of period $ 22.21 $ 12.97
Total Return 22.17% (17.39%)
Ratios to Average Net Assets    
Total income (loss) 20.05% (16.34%)
Management fees [1] 0.75% 0.75%
Total expenses excluding management fees [1] 0.31% 0.51%
Expenses waived [1] (0.16%) (0.36%)
Net expenses excluding management fees [1] 0.15% 0.15%
Net income (loss) 19.61% (16.79%)
[1] Annualized.