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CONSOLIDATED STATEMENTS OF CHANGES IN NET ASSETS (Parentheticals) - $ / shares | 12 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2014
Statement of Stockholders' Equity [Abstract] |  |  |  |  |  |  |  |  |  |
Net investment income (in USD per share) | $ 0.94 | $ 0.76 | $ 0.75 | $ 0.80 | $ 0.82 | $ 0.84 | $ 0.84 | $ 0.70 | $ 0.84 | $ 0.12
Return of capital (in USD per share) | $ 0.00 | $ 0.04 | $ 0.03 | $ 0.01 | $ 0.02 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0.72

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