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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Per Share Operating Performance: |  |  |  |  |
Net asset value, beginning of year |  | $ 9.61 | $ 12.43 | $ 9.20 | $ 16.91
Total income (loss) |  | 3.18 | (2.13) | 3.61 | (6.57)
Total expenses |  | (0.04) | (0.05) | (0.06) | (0.09)
Net increase (decrease) in net asset value |  | 3.14 | (2.18) | 3.55 | (6.66)
Net asset value, end of period |  | $ 12.75 | $ 10.25 | $ 12.75 | $ 10.25
Total Return |  | 32.67% | (17.54%) | 38.59% | (39.38%)
Ratios to Average Net Assets |  |  |  |  |
Total income (loss) |  | 29.45% | (18.27%) | 34.79% | (47.61%)
Management fees | [1] | 0.60% | 0.60% | 0.60% | 0.60%
Total expenses excluding management fees | [1] | 0.80% | 0.96% | 0.67% | 0.84%
Net income (loss) |  | 29.10% | (18.66%) | 34.16% | (48.33%)
[1] | # Annualized.

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