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INVESTMENTS - Schedule of Fair Value Measurements Using Significant Unobservable Inputs (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Total |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | $ 757,625 | $ 791,324 | $ 796,222 | $ 704,562
New investments, repayments and settlements: |  |  |  |
Issuances/originations | 74,031 | 47,846 | 268,103 | 153,183
Settlements/repayments | (80,934) | (85,084) | (238,417) | (121,397)
Net proceeds from sales | (731) | (1,434) | (89,564) | (2,663)
Transfers | 0 |  | 0 |
Ending balance | 746,214 | 757,535 | 746,214 | 757,535
Total | Net realized gain (loss) |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | (3,708) | 1,415 | 61,718 | 1,893
Total | Net unrealized appreciation (depreciation) and Reversal of prior period net depreciation (appreciation) on realization |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | (6,417) | 3,559 | 3,659 | 22,063
Reversal of prior period net depreciation (appreciation) on realization | 6,348 | (91) | (55,507) | (106)
Secured First Lien Debt |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | 541,524 | 563,849 | 554,937 | 510,701
New investments, repayments and settlements: |  |  |  |
Issuances/originations | 73,889 | 26,964 | 212,754 | 106,939
Settlements/repayments | (80,193) | (41,749) | (221,130) | (73,555)
Net proceeds from sales | 0 | 50 | 0 | 50
Transfers | (8,500) |  | (8,500) |
Ending balance | 529,149 | 542,283 | 529,149 | 542,283
Secured First Lien Debt | Net realized gain (loss) |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | (4,439) | (50) | (8,513) | (50)
Secured First Lien Debt | Net unrealized appreciation (depreciation) and Reversal of prior period net depreciation (appreciation) on realization |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | 815 | (6,748) | (10,011) | (1,750)
Reversal of prior period net depreciation (appreciation) on realization | 6,053 | (33) | 9,612 | (52)
Secured second lien debt |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | 144,970 | 140,409 | 113,716 | 127,854
New investments, repayments and settlements: |  |  |  |
Issuances/originations | 130 | 20,132 | 47,827 | 35,494
Settlements/repayments | (741) | (43,335) | (17,287) | (47,842)
Net proceeds from sales | 0 | 0 | 0 | 0
Transfers | 0 |  | 0 |
Ending balance | 144,299 | 117,875 | 144,299 | 117,875
Secured second lien debt | Net realized gain (loss) |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | 0 | 0 | 0 | 0
Secured second lien debt | Net unrealized appreciation (depreciation) and Reversal of prior period net depreciation (appreciation) on realization |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | (355) | 727 | (252) | 2,270
Reversal of prior period net depreciation (appreciation) on realization | 295 | (58) | 295 | 99
Unsecured debt |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | 335 | 33 | 32 | 24
New investments, repayments and settlements: |  |  |  |
Issuances/originations | 12 | 0 | 345 | 0
Settlements/repayments | 0 | 0 | 0 | 0
Net proceeds from sales | 0 | 0 | 0 | 0
Transfers | 0 |  | 0 |
Ending balance | 354 | 33 | 354 | 33
Unsecured debt | Net realized gain (loss) |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | 0 | 0 | 0 | 0
Unsecured debt | Net unrealized appreciation (depreciation) and Reversal of prior period net depreciation (appreciation) on realization |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | 7 | 0 | (23) | 9
Reversal of prior period net depreciation (appreciation) on realization | 0 | 0 | 0 | 0
Preferred Equity |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | 25,031 | 32,598 | 31,346 | 26,855
New investments, repayments and settlements: |  |  |  |
Issuances/originations | 0 | 0 | 6,262 | 10,000
Settlements/repayments | 0 | 0 | 0 | 0
Net proceeds from sales | 0 | 0 | (19,404) | (969)
Transfers | 0 |  | 0 |
Ending balance | 25,717 | 32,965 | 25,717 | 32,965
Preferred Equity | Net realized gain (loss) |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | 0 | 0 | 5,404 | 219
Preferred Equity | Net unrealized appreciation (depreciation) and Reversal of prior period net depreciation (appreciation) on realization |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | 686 | 367 | 7,513 | (3,270)
Reversal of prior period net depreciation (appreciation) on realization | 0 | 0 | (5,404) | 130
Common Equity/ Equivalents |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | 45,765 | 54,435 | 96,191 | 39,128
New investments, repayments and settlements: |  |  |  |
Issuances/originations | 0 | 750 | 915 | 750
Settlements/repayments | 0 | 0 | 0 | 0
Net proceeds from sales | (731) | (1,484) | (70,160) | (1,744)
Transfers | 8,500 |  | 8,500 |
Ending balance | 46,695 | 64,379 | 46,695 | 64,379
Common Equity/ Equivalents | Net realized gain (loss) |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | 731 | 1,465 | 64,827 | 1,724
Common Equity/ Equivalents | Net unrealized appreciation (depreciation) and Reversal of prior period net depreciation (appreciation) on realization |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Total gain (loss) | (7,570) | 9,213 | 6,432 | 24,804
Reversal of prior period net depreciation (appreciation) on realization | $ 0 | $ 0 | $ (60,010) | $ (283)

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