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SUBSEQUENT EVENTS - Narrative (Details) - USD ($) $ in Thousands |  | 12 Months Ended
--- | --- | ---
Nov. 13, 2023 | Sep. 30, 2023 | Sep. 30, 2022
Subsequent Event [Line Items] |  |  |  |  |
Cost |  | $ 722,269 | [1] | $ 656,053 | [2]
Line of credit commitment |  | $ 45,231 |  | $ 74,846 |
Subsequent Event | Quality Environmental Midco, Inc. |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Cost | $ 11,000 |  |  |  |
Line of credit commitment | $ 2,000 |  |  |  |
[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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