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FINANCIAL HIGHLIGHTS (Per share Performance Data and Other Supplemental Financial Data) (Detail) - $ / shares
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Per Share Operating Performance:    
Net asset value, beginning of period $ 12.08 $ 11.71
Total income (loss) 3.46 (1.21)
Total expenses (0.07) (0.06)
Net increase (decrease) in net asset value 3.39 (1.27)
Net asset value, end of period $ 15.47 $ 10.44
Total Return 28.06% (10.85%)
Ratios to Average Net Assets    
Total income (loss) 25.82% (8.25%)
Management fees [1] 0.45% 0.45%
Expenses excluding management fees [1] 0.27% 0.29%
Net income (loss) 25.27% (8.81%)
[1] Annualized.