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 |

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