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FINANCIAL HIGHLIGHTS (Per share Performance Data and Other Supplemental Financial Data) (Detail) - $ / shares
 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Per Share Operating Performance:    
Net asset value, beginning of period $ 11.71 $ 11.02
Total income (loss) (2.2) 0.58
Total expenses (0.04) (0.04)
Net increase (decrease) in net asset value (2.24) 0.54
Net asset value, end of period $ 9.47 $ 11.56
Total Return (19.13%) 4.90%
Ratios to Average Net Assets    
Total income (loss) (17.32%) 13.58%
Management fees [1] 0.45% 0.45%
Expenses excluding management fees [1] 0.30% 0.33%
Net income (loss) (17.69%) 13.19%
[1] Annualized.