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FINANCIAL HIGHLIGHTS (Per share Performance Data and Other Supplemental Financial Data) (Detail) - $ / shares
 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Per Share Operating Performance:    
Net asset value, beginning of period $ 12.08 $ 11.71
Total income (loss) 2.99 (2.20)
Total expenses (0.05) (0.04)
Net increase (decrease) in net asset value 2.94 (2.24)
Net asset value, end of period $ 15.02 $ 9.47
Total Return 24.34% (19.13%)
Ratios to Average Net Assets    
Total income (loss) 22.33% (17.32%)
Management fees [1] 0.45% 0.45%
Expenses excluding management fees [1] 0.28% 0.30%
Net income (loss) 21.97% (17.69%)
[1] Annualized.