Quarterly report [Sections 13 or 15(d)]

INVESTMENTS - Schedule of Fair Value Measurements Using Significant Unobservable Inputs (Details)

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INVESTMENTS - Schedule of Fair Value Measurements Using Significant Unobservable Inputs (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2026
Mar. 31, 2025
Total        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance $ 897,837 $ 794,460 $ 854,013 $ 796,222
New investments, repayments and settlements:        
Issuances/originations 45,552 46,490 146,852 194,072
Settlements/repayments (45,401) (65,387) (96,713) (157,483)
Net proceeds from sales (785) (15,752) (3,025) (88,833)
Ending balance 901,839 757,625 901,839 757,625
Total | Net realized gain (loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) (213) 7,702 1,465 65,426
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) 5,491 (2,042) 1,993 10,076
Reversal of prior period net depreciation (appreciation) on realization (642) (7,846) (2,746) (61,855)
Secured First Lien Debt        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 647,576 586,409 622,371 554,937
New investments, repayments and settlements:        
Issuances/originations 29,114 27,111 86,390 138,865
Settlements/repayments (44,697) (64,682) (76,464) (140,937)
Net proceeds from sales 0 0 0 0
Ending balance 630,527 541,524 630,527 541,524
Secured First Lien Debt | Net realized gain (loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) (213) 0 (325) (4,074)
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) (611) (7,292) (634) (10,826)
Reversal of prior period net depreciation (appreciation) on realization (642) (22) (811) 3,559
Secured second lien debt        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 170,662 127,234 150,542 113,716
New investments, repayments and settlements:        
Issuances/originations 16,342 18,453 55,413 47,697
Settlements/repayments (704) (705) (20,249) (16,546)
Net proceeds from sales 0 0 0 0
Ending balance 185,141 144,970 185,141 144,970
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) (1,159) (12) (420) 103
Reversal of prior period net depreciation (appreciation) on realization 0 0 (145) 0
Unsecured debt        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 346 359 333 32
New investments, repayments and settlements:        
Issuances/originations 13 11 26 333
Settlements/repayments 0 0 0 0
Net proceeds from sales 0 0 0 0
Ending balance 341 335 341 335
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) (18) (35) (18) (30)
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 36,761 31,294 31,214 31,346
New investments, repayments and settlements:        
Issuances/originations 0 0 4,940 6,262
Settlements/repayments 0 0 0 0
Net proceeds from sales 0 (9,954) 0 (19,404)
Ending balance 37,394 25,031 37,394 25,031
Preferred Equity | Net realized gain (loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) 0 2,954 0 5,404
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) 633 3,691 1,240 6,827
Reversal of prior period net depreciation (appreciation) on realization 0 (2,954) 0 (5,404)
Common Equity/ Equivalents        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 42,492 49,164 49,553 96,191
New investments, repayments and settlements:        
Issuances/originations 83 915 83 915
Settlements/repayments 0 0 0 0
Net proceeds from sales (785) (5,798) (3,025) (69,429)
Ending balance 48,436 45,765 48,436 45,765
Common Equity/ Equivalents | Net realized gain (loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) 0 4,748 1,790 64,096
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) 6,646 1,606 1,825 14,002
Reversal of prior period net depreciation (appreciation) on realization $ 0 $ (4,870) $ (1,790) $ (60,010)