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FINANCIAL HIGHLIGHTS (Per share Performance Data and Other Supplemental Financial Data) (Detail) - $ / shares
 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Per Share Operating Performance:    
Net asset value, beginning of period $ 11.02 $ 20.15
Total income (loss) 0.58 (0.15)
Total expenses (0.04) (0.07)
Net increase (decrease) in net asset value 0.54 (0.22)
Net asset value, end of period $ 11.56 $ 19.93
Total Return 4.90% (1.09%)
Ratios to Average Net Assets    
Total income (loss) 13.58% 9.69%
Management fees [1] 0.45% 0.45%
Expenses excluding management fees [1] 0.33% 0.26%
Net income (loss) 13.19% 9.34%
[1] Annualized.