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FINANCIAL HIGHLIGHTS (Per share Performance Data and Other Supplemental Financial Data) (Detail) - $ / shares | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Per Share Operating Performance: |  |  |
Net asset value, beginning of year | $ 12.08 | $ 11.71 | $ 11.02
Total income (loss) | (2.39) | 0.45 | 0.77
Total expenses | (0.10) | (0.08) | (0.08)
Net increase (decrease) in net asset value | (2.49) | 0.37 | 0.69
Net asset value, end of year | $ 9.59 | $ 12.08 | $ 11.71
Total Return | (20.61%) | 3.16% | 6.26%
Ratios to Average Net Assets |  |  |
Total income (loss) | (15.36%) | 3.75% | 20.57%
Management fees | 0.45% | 0.45% | 0.45%
Expenses excluding management fees | 0.28% | 0.31% | 0.32%
Net income (loss) | (16.09%) | 2.99% | 19.80%

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