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Note 9 - Concentrations of Credit Risk (Details Textual) - Debt Investments [Member] - Five Largest Debt Investments [Member] | 3 Months Ended | 6 Months Ended | 12 Months Ended
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Jun. 30, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022 | Dec. 31, 2022
Investment Concentration Risk [Member] |  |  |  |  |
Concentration Risk, Percentage |  |  | 25.00% |  | 23.00%
Investment Concentration Risk [Member] | Reported Value Measurement [Member] |  |  |  |  |
Concentration Risk, Percentage |  |  | 23.00% | 22.00% |
Investment Concentration Risk [Member] | Estimate of Fair Value Measurement [Member] |  |  |  |  |
Concentration Risk, Percentage | 23.00% | 23.00% | 23.00% | 22.00% |
Investment Income Concentration Risk [Member] | Reported Value Measurement [Member] |  |  |  |  |
Concentration Risk, Percentage | 23.00% | 22.00% |  |  |

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