v3.23.3
Portfolio Investments - Schedule of Financial Instruments Disclosed, but Carried at Fair Value (Details) - USD ($)
$ in Thousands
Sep. 30, 2023
Sep. 30, 2022
Schedule of Investments [Line Items]    
Carrying Value $ 1,600,731 $ 1,301,043
Fair Value 1,588,201 1,277,105
Syndicated Facility    
Schedule of Investments [Line Items]    
Carrying Value 430,000 540,000
Fair Value 430,000 540,000
OSI2 Citibank Facility    
Schedule of Investments [Line Items]    
Carrying Value 280,000  
Fair Value 280,000  
Citibank Facility    
Schedule of Investments [Line Items]    
Carrying Value   160,000
Fair Value   160,000
2025 Notes    
Schedule of Investments [Line Items]    
Carrying Value 298,241 296,991
Fair Value 286,437 283,077
2027 Notes    
Schedule of Investments [Line Items]    
Carrying Value 306,412 304,052
Fair Value 301,784 294,028
2029 Notes    
Schedule of Investments [Line Items]    
Carrying Value 286,078  
Fair Value 289,980  
Level 1    
Schedule of Investments [Line Items]    
Fair Value 0 0
Level 1 | Syndicated Facility    
Schedule of Investments [Line Items]    
Fair Value 0 0
Level 1 | OSI2 Citibank Facility    
Schedule of Investments [Line Items]    
Fair Value 0  
Level 1 | Citibank Facility    
Schedule of Investments [Line Items]    
Fair Value   0
Level 1 | 2025 Notes    
Schedule of Investments [Line Items]    
Fair Value 0 0
Level 1 | 2027 Notes    
Schedule of Investments [Line Items]    
Fair Value 0 0
Level 1 | 2029 Notes    
Schedule of Investments [Line Items]    
Fair Value 0  
Level 2    
Schedule of Investments [Line Items]    
Fair Value 878,201 577,105
Level 2 | Syndicated Facility    
Schedule of Investments [Line Items]    
Fair Value 0 0
Level 2 | OSI2 Citibank Facility    
Schedule of Investments [Line Items]    
Fair Value 0  
Level 2 | Citibank Facility    
Schedule of Investments [Line Items]    
Fair Value   0
Level 2 | 2025 Notes    
Schedule of Investments [Line Items]    
Fair Value 286,437 283,077
Level 2 | 2027 Notes    
Schedule of Investments [Line Items]    
Fair Value 301,784 294,028
Level 2 | 2029 Notes    
Schedule of Investments [Line Items]    
Fair Value 289,980  
Level 3    
Schedule of Investments [Line Items]    
Fair Value 710,000 700,000
Level 3 | Syndicated Facility    
Schedule of Investments [Line Items]    
Fair Value 430,000 540,000
Level 3 | OSI2 Citibank Facility    
Schedule of Investments [Line Items]    
Fair Value 280,000  
Level 3 | Citibank Facility    
Schedule of Investments [Line Items]    
Fair Value   160,000
Level 3 | 2025 Notes    
Schedule of Investments [Line Items]    
Fair Value 0 0
Level 3 | 2027 Notes    
Schedule of Investments [Line Items]    
Fair Value 0 $ 0
Level 3 | 2029 Notes    
Schedule of Investments [Line Items]    
Fair Value $ 0