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FINANCIAL HIGHLIGHTS (Per share Performance Data and Other Supplemental Financial Data) (Detail) - $ / shares
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Per Share Operating Performance:    
Net asset value, beginning of year $ 11.71 $ 11.02
Total income (loss) (1.07) (1.26)
Total expenses (0.02) (0.02)
Net increase (decrease) in net asset value (1.09) (1.28)
Net asset value, end of year $ 10.62 $ 9.74
Total Return (9.31%) (11.62%)
Ratios to Average Net Assets    
Total income (loss) (8.52%) (4.91%)
Management fees [1] 0.45% 0.45%
Expenses excluding management fees [1] 0.28% 0.37%
Net income (loss) (8.71%) (5.11%)
[1] Annualized.