v3.25.2
Portfolio Investments - Schedule of Financial Instruments Disclosed, but Carried at Fair Value (Details) - USD ($)
$ in Thousands
Jun. 30, 2025
Sep. 30, 2024
Schedule of Investments [Line Items]    
Carrying Value $ 1,447,551 $ 1,638,693
Fair Value 1,455,883 1,648,133
Syndicated Facility payable    
Schedule of Investments [Line Items]    
Carrying Value 510,000 430,000
Fair Value 510,000 430,000
OSI2 Citibank Facility payable    
Schedule of Investments [Line Items]    
Carrying Value   280,000
Fair Value   280,000
2025 Notes    
Schedule of Investments [Line Items]    
Carrying Value   299,492
Fair Value   298,146
2027 Notes    
Schedule of Investments [Line Items]    
Carrying Value 333,558 327,612
Fair Value 335,713 327,723
2029 Notes    
Schedule of Investments [Line Items]    
Carrying Value 299,613 301,589
Fair Value 309,210 312,264
2030 Notes    
Schedule of Investments [Line Items]    
Carrying Value 304,380  
Fair Value 300,960  
Level 1    
Schedule of Investments [Line Items]    
Fair Value 0 0
Level 1 | Syndicated Facility payable    
Schedule of Investments [Line Items]    
Fair Value 0 0
Level 1 | OSI2 Citibank Facility payable    
Schedule of Investments [Line Items]    
Fair Value   0
Level 1 | 2025 Notes    
Schedule of Investments [Line Items]    
Fair Value   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 0
Level 1 | 2030 Notes    
Schedule of Investments [Line Items]    
Fair Value 0  
Level 2    
Schedule of Investments [Line Items]    
Fair Value 945,883 938,133
Level 2 | Syndicated Facility payable    
Schedule of Investments [Line Items]    
Fair Value 0 0
Level 2 | OSI2 Citibank Facility payable    
Schedule of Investments [Line Items]    
Fair Value   0
Level 2 | 2025 Notes    
Schedule of Investments [Line Items]    
Fair Value   298,146
Level 2 | 2027 Notes    
Schedule of Investments [Line Items]    
Fair Value 335,713 327,723
Level 2 | 2029 Notes    
Schedule of Investments [Line Items]    
Fair Value 309,210 312,264
Level 2 | 2030 Notes    
Schedule of Investments [Line Items]    
Fair Value 300,960  
Level 3    
Schedule of Investments [Line Items]    
Fair Value 510,000 710,000
Level 3 | Syndicated Facility payable    
Schedule of Investments [Line Items]    
Fair Value 510,000 430,000
Level 3 | OSI2 Citibank Facility payable    
Schedule of Investments [Line Items]    
Fair Value   280,000
Level 3 | 2025 Notes    
Schedule of Investments [Line Items]    
Fair Value   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 $ 0
Level 3 | 2030 Notes    
Schedule of Investments [Line Items]    
Fair Value $ 0