Portfolio Investments - Schedule of Financial Instruments Disclosed, but Carried at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Schedule of Investments [Line Items]
Carrying Value	$ 1,486,880	$ 1,638,693
Fair Value	1,500,411	1,648,133
Syndicated Facility payable
Schedule of Investments [Line Items]
Carrying Value	545,000	430,000
Fair Value	545,000	430,000
OSI2 Citibank Facility
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	336,601	327,612
Fair Value	339,763	327,723
2029 Notes
Schedule of Investments [Line Items]
Carrying Value	300,460	301,589
Fair Value	314,520	312,264
2030 Notes
Schedule of Investments [Line Items]
Carrying Value	304,819
Fair Value	301,128
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
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	955,411	938,133
Level 2 | Syndicated Facility payable
Schedule of Investments [Line Items]
Fair Value	0	0
Level 2 | OSI2 Citibank Facility
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	339,763	327,723
Level 2 | 2029 Notes
Schedule of Investments [Line Items]
Fair Value	314,520	312,264
Level 2 | 2030 Notes
Schedule of Investments [Line Items]
Fair Value	301,128
Level 3
Schedule of Investments [Line Items]
Fair Value	545,000	710,000
Level 3 | Syndicated Facility payable
Schedule of Investments [Line Items]
Fair Value	545,000	430,000
Level 3 | OSI2 Citibank Facility
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