Annual report pursuant to Section 13 and 15(d)

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

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INVESTMENTS - Fair Value, Assets Measured on Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2023
Sep. 30, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value $ 704,584 $ 643,218
Leeds Acquisition Holdings, LLC    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 200 6,400
Funko Acquisition Holdings, LLC    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 22 58
Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 22 58
Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 704,562 643,160
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 704,562 643,160
Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 663,291 568,356
Significant Unobservable Inputs (Level 3) | Affiliate investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 10,421 39,091
Significant Unobservable Inputs (Level 3) | Control Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 30,850 35,713
Secured first lien debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 510,701 463,858
Secured first lien debt | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Secured first lien debt | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Secured first lien debt | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 510,701 463,858
Secured first lien debt | Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 491,686 413,631
Secured first lien debt | Significant Unobservable Inputs (Level 3) | Affiliate investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 2,895 34,804
Secured first lien debt | Significant Unobservable Inputs (Level 3) | Control Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 16,120 15,423
Secured second lien debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 127,854 115,928
Secured second lien debt | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Secured second lien debt | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Secured second lien debt | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 127,854 115,928
Secured second lien debt | Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 120,429 108,263
Secured second lien debt | Significant Unobservable Inputs (Level 3) | Control Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 7,425 7,665
Unsecured debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 24 55
Unsecured debt | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Unsecured debt | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Unsecured debt | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 24 55
Unsecured debt | Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 24 55
Preferred equity    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 26,855 27,046
Preferred equity | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Preferred equity | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Preferred equity | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 26,855 27,046
Preferred equity | Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 21,733 17,719
Preferred equity | Significant Unobservable Inputs (Level 3) | Affiliate investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 5,122 3,640
Preferred equity | Significant Unobservable Inputs (Level 3) | Control Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 5,687
Common equity/equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 39,150 36,331
Common equity/equivalents | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Common equity/equivalents | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 22 58
Common equity/equivalents | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 39,128 36,273
Common equity/equivalents | Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 29,419 28,688
Common equity/equivalents | Significant Unobservable Inputs (Level 3) | Affiliate investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 2,404 647
Common equity/equivalents | Significant Unobservable Inputs (Level 3) | Control Investments | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value $ 7,305 $ 6,938