Quarterly report [Sections 13 or 15(d)]

INVESTMENTS - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details)

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INVESTMENTS - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Mar. 31, 2025
Mar. 31, 2024
Total        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance $ 794,460 $ 749,717 $ 796,222 $ 704,562
New investments, repayments and settlements:        
Issuances/originations 46,490 46,128 194,072 105,337
Settlements/repayments (65,387) (14,470) (157,483) (36,313)
Net proceeds from sales (15,752) (969) (88,833) (1,229)
Ending balance 757,625 791,324 757,625 791,324
Total | Debt and Equity Securities, Realized Gain (Loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) 7,702 219 65,426 478
Total | Debt and Equity Securities, Unrealized Gain (Loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) (2,042) 10,431 10,076 18,504
Reversal of prior period net depreciation (appreciation) on realization (7,846) 268 (61,855) (15)
Secured first lien debt        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 586,409 546,602 554,937 510,701
New investments, repayments and settlements:        
Issuances/originations 27,111 29,951 138,865 79,975
Settlements/repayments (64,682) (10,687) (140,937) (31,806)
Net proceeds from sales 0 0 0 0
Ending balance 541,524 563,849 541,524 563,849
Secured first lien debt | Debt and Equity Securities, Realized Gain (Loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) 0 0 (4,074) 0
Secured first lien debt | Debt and Equity Securities, Unrealized Gain (Loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) (7,292) (1,998) (10,826) 4,998
Reversal of prior period net depreciation (appreciation) on realization (22) (19) 3,559 (19)
Secured second lien debt        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 127,234 126,571 113,716 127,854
New investments, repayments and settlements:        
Issuances/originations 18,453 15,177 47,697 15,362
Settlements/repayments (705) (3,783) (16,546) (4,507)
Net proceeds from sales 0 0 0 0
Ending balance 144,970 140,409 144,970 140,409
Secured second lien debt | Debt and Equity Securities, 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 | Debt and Equity Securities, Unrealized Gain (Loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) (12) 2,287 103 1,543
Reversal of prior period net depreciation (appreciation) on realization 0 157 0 157
Unsecured debt        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 359 23 32 24
New investments, repayments and settlements:        
Issuances/originations 11 0 333 0
Settlements/repayments 0 0 0 0
Net proceeds from sales 0 0 0 0
Ending balance 335 33 335 33
Unsecured debt | Debt and Equity Securities, 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 | Debt and Equity Securities, Unrealized Gain (Loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) (35) 10 (30) 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 31,294 29,722 31,346 26,855
New investments, repayments and settlements:        
Issuances/originations 0 1,000 6,262 10,000
Settlements/repayments 0 0 0 0
Net proceeds from sales (9,954) (969) (19,404) (969)
Ending balance 25,031 32,598 25,031 32,598
Preferred Equity | Debt and Equity Securities, Realized Gain (Loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) 2,954 219 5,404 219
Preferred Equity | Debt and Equity Securities, Unrealized Gain (Loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) 3,691 2,496 6,827 (3,637)
Reversal of prior period net depreciation (appreciation) on realization (2,954) 130 (5,404) 130
Common Equity/ Equivalents        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 49,164 46,799 96,191 39,128
New investments, repayments and settlements:        
Issuances/originations 915 0 915 0
Settlements/repayments 0 0 0 0
Net proceeds from sales (5,798) 0 (69,429) (260)
Ending balance 45,765 54,435 45,765 54,435
Common Equity/ Equivalents | Debt and Equity Securities, Realized Gain (Loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) 4,748 0 64,096 259
Common Equity/ Equivalents | Debt and Equity Securities, Unrealized Gain (Loss)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gain (loss) 1,606 7,636 14,002 15,591
Reversal of prior period net depreciation (appreciation) on realization $ (4,870) $ 0 $ (60,010) $ (283)