Quarterly report pursuant to Section 13 or 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
Jun. 30, 2024
Sep. 30, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value $ 757,535 $ 704,584
Leeds Acquisition Holdings, LLC    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 200 200
Funko Acquisition Holdings, LLC    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value   22
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 0 22
Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 757,535 704,562
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 757,535 704,562
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 710,051 663,291
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 9,892 10,421
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 37,592 30,850
Secured first lien debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 542,283 510,701
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 542,283 510,701
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 527,215 491,686
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 975 2,895
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 14,093 16,120
Secured second lien debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 117,875 127,854
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 117,875 127,854
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 110,610 120,429
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 7,265 7,425
Unsecured debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 33 24
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 33 24
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 33 24
Preferred equity    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 32,965 26,855
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 32,965 26,855
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 26,931 21,733
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 6,034 5,122
Common equity/equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 64,379 39,150
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 0 22
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 64,379 39,128
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 45,262 29,419
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 2,883 2,404
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 $ 16,234 $ 7,305