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FINANCIAL HIGHLIGHTS (Per share Performance Data and Other Supplemental Financial Data) (Detail) - $ / shares
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Per Share Operating Performance:    
Net asset value, beginning of period $ 12.08 $ 11.71
Total income (loss) 1.04 (1.07)
Total expenses (0.02) (0.02)
Net increase (decrease) in net asset value 1.02 (1.09)
Net asset value, end of period $ 13.1 $ 10.62
Total Return 8.44% (9.31%)
Ratios to Average Net Assets    
Total income (loss) 8.85% (8.52%)
Management fees [1] 0.45% 0.45%
Expenses excluding management fees [1] 0.29% 0.28%
Net income (loss) 8.67% (8.71%)
[1] Annualized.