Borrowings - 2018 Debt Securitization (Narrative) (Details) $ in Thousands		12 Months Ended
	Nov. 16, 2018 USD ($)	Sep. 30, 2025 USD ($)		Sep. 30, 2024 USD ($) portfolioCompany		Sep. 30, 2023 USD ($)
Debt Instrument [Line Items]
Investments, at fair value		$ 8,769,389	[1]	$ 8,235,411	[2]
2018 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount	$ 602,400
Debt instrument, basis spread on variable rate		0.26161%
Repayments of debt				$ 158,712		$ 19,503
Number of investment portfolio company | portfolioCompany				44
Investments, at fair value				$ 351,446
2018 Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount	113,400
2018 Debt Securitization | AAA/AAA Class A 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 327,000
Debt instrument, basis spread on variable rate	1.48%
2018 Debt Securitization | AA Class B 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 61,200
Debt instrument, basis spread on variable rate	2.10%
2018 Debt Securitization | A Class C-1 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 20,000
Debt instrument, basis spread on variable rate	2.80%
2018 Debt Securitization | A Class C-2 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 38,800
Debt instrument, basis spread on variable rate	2.65%
2018 Debt Securitization | BBB- Class D 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 42,000
Debt instrument, basis spread on variable rate	2.95%

[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.