Annual report pursuant to Section 13 and 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
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Total    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance $ 643,160 $ 551,047
New investments, repayments and settlements:    
Issuances/originations 176,760 279,430
Settlements/repayments (105,203) (159,412)
Net proceeds from sales (12,332) (15,714)
Transfers 0 0
Ending balance 704,562 643,160
Total | Debt and Equity Securities, Realized Gain (Loss)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gain (loss) 8,214 5,380
Total | Debt and Equity Securities, Unrealized Gain (Loss)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gain (loss) 1,743 (16,540)
Reversal of prior period net depreciation (appreciation) on realization (7,780) (1,031)
Secured first lien debt    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 463,858 337,394
New investments, repayments and settlements:    
Issuances/originations 154,762 204,970
Settlements/repayments (101,085) (77,898)
Net proceeds from sales 0 1,339
Transfers 0 (6,750)
Ending balance 510,701 463,858
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) (107) (8,471)
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) (7,577) (693)
Reversal of prior period net depreciation (appreciation) on realization 850 13,967
Secured second lien debt    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 115,928 135,956
New investments, repayments and settlements:    
Issuances/originations 15,421 62,570
Settlements/repayments (4,118) (81,264)
Net proceeds from sales 0 0
Transfers 0 0
Ending balance 127,854 115,928
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
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) 617 267
Reversal of prior period net depreciation (appreciation) on realization 6 (1,601)
Unsecured debt    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 55 10
New investments, repayments and settlements:    
Issuances/originations 0 25
Settlements/repayments 0 0
Net proceeds from sales 0 0
Transfers 0 0
Ending balance 24 55
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) (95) (25)
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) (31) 20
Reversal of prior period net depreciation (appreciation) on realization 95 25
Preferred equity    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 27,046 29,246
New investments, repayments and settlements:    
Issuances/originations 2,045 6,846
Settlements/repayments 0 (250)
Net proceeds from sales (654) (1,321)
Transfers 0 0
Ending balance 26,855 27,046
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) (279) 0
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) (1,829) (3,166)
Reversal of prior period net depreciation (appreciation) on realization 526 (4,309)
Common equity/equivalents    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 36,273 48,441
New investments, repayments and settlements:    
Issuances/originations 4,532 5,019
Settlements/repayments 0 0
Net proceeds from sales (11,678) (15,732)
Transfers 0 6,750
Ending balance 39,128 36,273
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) 8,695 13,876
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) 10,563 (12,968)
Reversal of prior period net depreciation (appreciation) on realization $ (9,257) $ (9,113)