Borrowings - 2024 Debt Securitization (Narrative) (Details) $ in Thousands		12 Months Ended
	Nov. 18, 2024 USD ($)	Sep. 30, 2025 USD ($) portfolioCompany		Sep. 30, 2024 USD ($)	[2]
Debt Instrument [Line Items]
Investments, at fair value		$ 8,769,389	[1]	$ 8,235,411
2024 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount	$ 2,200,500
Debt instrument, basis spread on variable rate		4.30%
Number of investment portfolio company | portfolioCompany		126
Investments, at fair value		$ 2,161,579
2024 Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount	517,500
2024 Debt Securitization | AAA/AAA Class A -1R 2024 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 1,192,400
Debt instrument, basis spread on variable rate	1.56%
2024 Debt Securitization | AAA Class A-2RR 2024 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 171,600
Debt instrument, basis spread on variable rate	1.75%
2024 Debt Securitization | AA Class B-R 2024 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 165,000
Debt instrument, basis spread on variable rate	1.70%
2024 Debt Securitization | A Class C-R 2024 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 154,000
Debt instrument, basis spread on variable rate	2.10%

[1] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.

[2] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.