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FINANCIAL HIGHLIGHTS (Details) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Per Share Operating Performance:      
Net asset value, beginning of year $ 15.7 $ 12.22 $ 22.39
Total income (loss) 2.61 3.6 (10)
Net expenses (0.13) (0.12) (0.17)
Net increase (decrease) in net asset value 2.48 3.48 (10.17)
Net asset value, end of year $ 18.18 $ 15.7 $ 12.22
Total Return 15.80% 28.48% (45.42%)
Ratios to Average Net Assets      
Total income (loss) 16.22% 34.23% (46.52%)
Management fees 0.75% 0.75% 0.75%
Total expenses excluding management fees 0.48% 0.32% 0.30%
Expenses waived (0.33%) (0.17%) (0.15%)
Net expenses excluding management fees 0.15% 0.15% 0.15%
Net income (loss) 15.32% 33.33% (47.42%)