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FINANCIAL HIGHLIGHTS (Details) - $ / shares
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Per Share Operating Performance:    
Net asset value, beginning of year $ 15.18 $ 18.18
Total income (loss) 4.08 0.99
Net expenses (0.04) (0.04)
Net increase (decrease) in net asset value 4.04 0.95
Net asset value, end of year $ 19.22 $ 19.13
Total Return 26.61% 5.23%
Ratios to Average Net Assets    
Total income (loss) 22.38% 5.04%
Management fees [1] 0.75% 0.75%
Total expenses excluding management fees [1] 0.18% 0.40%
Expenses waived [1] (0.03%) (0.25%)
Net expenses excluding management fees [1] 0.15% 0.15%
Net income (loss) 22.16% 4.82%
[1] Annualized