Portfolio Investments - Schedule of Financial Instruments Disclosed, but Carried at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Sep. 30, 2025
Schedule of Investments [Line Items]
Carrying Value	$ 1,438,842	$ 1,486,880
Fair Value	1,450,652	1,500,411
Syndicated Facility payable
Schedule of Investments [Line Items]
Carrying Value	501,000	545,000
Fair Value	501,000	545,000
2027 Notes
Schedule of Investments [Line Items]
Carrying Value	343,513	336,601
Fair Value	345,338	339,763
2029 Notes
Schedule of Investments [Line Items]
Carrying Value	295,816	300,460
Fair Value	305,811	314,520
2030 Notes
Schedule of Investments [Line Items]
Carrying Value	298,513	304,819
Fair Value	298,503	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 \| 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	0
Level 2
Schedule of Investments [Line Items]
Fair Value	949,652	955,411
Level 2 \| Syndicated Facility payable
Schedule of Investments [Line Items]
Fair Value	0	0
Level 2 \| 2027 Notes
Schedule of Investments [Line Items]
Fair Value	345,338	339,763
Level 2 \| 2029 Notes
Schedule of Investments [Line Items]
Fair Value	305,811	314,520
Level 2 \| 2030 Notes
Schedule of Investments [Line Items]
Fair Value	298,503	301,128
Level 3
Schedule of Investments [Line Items]
Fair Value	501,000	545,000
Level 3 \| Syndicated Facility payable
Schedule of Investments [Line Items]
Fair Value	501,000	545,000
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	$ 0