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INVESTMENTS - Fair Value Measurement Inputs and Valuation Techniques (Details) $ in Thousands | Mar. 31, 2025 USD ($) | Sep. 30, 2024 USD ($)
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Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | $ 762,636 | $ 796,260 | [1]
Investments at fair value | 762,636 | 796,260 |
Gladstone Alternative |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments at fair value | 5,000 |  |
Leeds Acquisition Holdings L L C |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 36,000 | 38,000 |
Investments at fair value | 36,000 | 38,000 |
Secured first lien debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments at fair value | 541,524 | 554,937 |
Secured second lien debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments at fair value | 144,970 | 113,716 |
Unsecured debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments at fair value | 335 | 32 |
Significant Unobservable Inputs (Level 3) |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 757,625 | 796,222 |
Investments at fair value | 757,625 | 796,222 |
Significant Unobservable Inputs (Level 3) | Secured first lien debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments at fair value | 541,524 | 554,937 |
Significant Unobservable Inputs (Level 3) | Secured second lien debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments at fair value | 144,970 | 113,716 |
Significant Unobservable Inputs (Level 3) | Unsecured debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments at fair value | 335 | 32 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Secured first lien debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 467,857 | 464,090 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Secured second lien debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | $ 132,790 | $ 101,928 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.099 | 0.108 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.175 | 0.173 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.117 | 0.126 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.117 | 0.122 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.154 | 0.160 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.133 | 0.141 |
Significant Unobservable Inputs (Level 3) | Market Quote | Secured second lien debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | $ 3,388 | $ 3,241 |
Significant Unobservable Inputs (Level 3) | Market Quote | IBP | Secured second lien debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.920 | 0.880 |
Significant Unobservable Inputs (Level 3) | Market Quote | IBP | Secured second lien debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.920 | 0.880 |
Significant Unobservable Inputs (Level 3) | Market Quote | IBP | Secured second lien debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.920 | 0.880 |
Significant Unobservable Inputs (Level 3) | TEV | Secured first lien debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | $ 73,667 | $ 90,847 |
Significant Unobservable Inputs (Level 3) | TEV | Secured second lien debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 8,792 | 8,547 |
Significant Unobservable Inputs (Level 3) | TEV | Unsecured debt |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 335 | 32 |
Significant Unobservable Inputs (Level 3) | TEV | Preferred and common equity / equivalents |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | $ 70,796 | $ 127,537 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 5.5 | 4.1 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 7.1 | 13.9 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 6.2 | 10 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 5.4 | 5.4 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 5.4 | 5.4 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 5.4 | 5.4 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Unsecured debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 7.1 | 0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Unsecured debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 7.1 | 0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Unsecured debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 7.1 | 0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 5 | 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] |  |  |
Investment owned, measurement input | 14.5 | 13.9 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 7.8 | 8 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | $ 2,504 | $ 3,020 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 4,222 | 16,211 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 3,355 | 10,309 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 2,543 | 3,343 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 2,543 | 3,343 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 2,543 | 3,343 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Unsecured debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 4,222 | 0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Unsecured debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 4,222 | 0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Unsecured debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 4,222 | 0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 1,318 | 1,182 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 144,799 | 144,458 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | $ 10,694 | $ 10,847 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.4 | 0.2 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.7 | 4.6 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.6 | 2.1 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 1 | 1 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 1 | 1 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 1 | 1 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.4 | 0.2 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 1 | 4.6 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investment owned, measurement input | 0.5 | 2 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | $ 10,667 | $ 6,336 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 21,318 | 21,118 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 15,691 | 13,981 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 5,616 | 7,834 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 5,616 | 7,834 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 5,616 | 7,834 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 5,402 | 4,672 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | 21,318 | 21,118 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Weighted Average |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Investments owned, fair value | $ 10,537 | $ 12,587 |
[1] | Cumulative gross unrealized depreciation for federal income tax purposes is $66.1 million; cumulative gross unrealized appreciation for federal income tax purposes is $85.8 million. Cumulative net unrealized appreciation is $19.7 million, based on a tax cost of $776.5 million.

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