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INVESTMENTS - Fair Value Measurement Inputs and Valuation Techniques (Details) $ in Thousands | Sep. 30, 2023 USD ($) | Sep. 30, 2022 USD ($) investment | Sep. 30, 2021 USD ($)
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Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | $ 704,815 | [1] | $ 649,615 | [2] |
Leeds Acquisition Holdings, LLC |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value |  |  | 200 |  | $ 6,400
Funko Acquisition Holdings, LLC |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value |  |  | 22 |  | $ 58
Significant Unobservable Inputs (Level 3) |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 704,562 |  | 643,160 |  |
Significant Unobservable Inputs (Level 3) | Measurement Input, Expected Payoff Amount | Secured first lien debt |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value |  |  | $ 18,800 |  |
Investment owned, number of investments | investment |  |  | 1 |  |
Significant Unobservable Inputs (Level 3) | Measurement Input, Expected Payoff Amount | Preferred and common equity / equivalents |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value |  |  | $ 8,000 |  |
Investment owned, number of investments | investment |  |  | 1 |  |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Secured first lien debt |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 461,638 |  | $ 423,912 |  |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Secured second lien debt |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | $ 110,820 |  | $ 97,472 |  |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.118 |  | 0.080 |  |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.299 |  | 0.295 |  |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.148 |  | 0.118 |  |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.125 |  | 0.115 |  |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.156 |  | 0.154 |  |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.145 |  | 0.138 |  |
Significant Unobservable Inputs (Level 3) | TEV | Secured first lien debt |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | $ 49,063 |  | $ 39,946 |  |
Significant Unobservable Inputs (Level 3) | TEV | Secured second lien debt |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 7,425 |  | 7,665 |  |
Significant Unobservable Inputs (Level 3) | TEV | Unsecured debt |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 24 |  | 55 |  |
Significant Unobservable Inputs (Level 3) | TEV | Preferred and common equity / equivalents |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | $ 65,983 |  | $ 63,319 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 4.7 |  | 4.8 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 6.8 |  | 6.7 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 6.7 |  | 6.2 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 5.6 |  | 5.6 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 5.6 |  | 5.6 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 5.6 |  | 5.6 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 4.7 |  | 4.1 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 13.0 |  | 11.0 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 6.9 |  | 6.5 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | $ 995 |  | $ 800 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 14,002 |  | 10,257 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 13,624 |  | 7,605 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 3,690 |  | 3,299 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 3,690 |  | 3,299 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 3,690 |  | 3,299 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 995 |  | 800 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 112,841 |  | 74,512 |  |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | $ 10,570 |  | $ 11,742 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.3 |  | 0.3 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.8 |  | 0.6 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.6 |  | 0.5 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 1.0 |  | 0.3 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 1.0 |  | 1.3 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 1.0 |  | 1.0 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.3 |  | 0.3 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 3.0 |  | 4.4 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 1.2 |  | 1.4 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | $ 14,934 |  | $ 11,514 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 16,283 |  | 16,320 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 15,361 |  | 14,656 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 5,044 |  | 764 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 5,044 |  | 11,514 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 5,044 |  | 4,249 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 4,213 |  | 764 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 16,283 |  | 42,926 |  |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | 14,959 |  | 19,963 |  |
Significant Unobservable Inputs (Level 3) | Market Quote | Secured second lien debt |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Investments, at fair value | $ 9,609 |  | $ 10,791 |  |
Significant Unobservable Inputs (Level 3) | Market Quote | IBP | Secured second lien debt | Minimum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.678 |  | 0.820 |  |
Significant Unobservable Inputs (Level 3) | Market Quote | IBP | Secured second lien debt | Maximum |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.940 |  | 0.950 |  |
Significant Unobservable Inputs (Level 3) | Market Quote | IBP | Secured second lien debt | Weighted Average |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Fair value, measurement input | 0.822 |  | 0.865 |  |
[1] | Cumulative gross unrealized depreciation for federal income tax purposes is $56.9 million; cumulative gross unrealized appreciation for federal income tax purposes is $33.7 million. Cumulative net unrealized depreciation is $23.2 million, based on a tax cost of $728.0 million.
[2] | Cumulative gross unrealized depreciation for federal income tax purposes is $52.3 million; cumulative gross unrealized appreciation for federal income tax purposes is $35.2 million. Cumulative net unrealized depreciation is $17.1 million, based on a tax cost of $666.7 million.

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