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FINANCIAL HIGHLIGHTS (Per share Performance Data and Other Supplemental Financial Data) (Detail) - $ / shares | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Per Share Operating Performance: |  |  |
Net asset value, beginning of year | $ 11.71 | $ 11.02 | $ 20.15
Total income (loss) | 0.45 | 0.77 | (9.01)
Total expenses | (0.08) | (0.08) | (0.12)
Net increase (decrease) in net asset value | 0.37 | 0.69 | (9.13)
Net asset value, end of year | $ 12.08 | $ 11.71 | $ 11.02
Total Return | 3.16% | 6.26% | (45.31%)
Ratios to Average Net Assets |  |  |
Total income (loss) | 3.75% | 20.57% | (49.08%)
Management fees | 0.45% | 0.45% | 0.45%
Expenses excluding management fees | 0.31% | 0.32% | 0.29%
Net income (loss) | 2.99% | 19.80% | (49.82%)

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