Investments - Fair Value, Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2023		Sep. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	$ 5,307,168		$ 5,372,788
Amortized Cost	5,510,452		5,593,836
Fair Value	5,443,427	[1]	5,516,613	[2]
Senior secured
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	457,493		518,721
Amortized Cost	461,296		527,449
Fair Value	438,837		503,985
One stop
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	4,804,977		4,812,987
Amortized Cost	4,738,462		4,763,128
Fair Value	4,662,636		4,678,099
Second lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	35,452		32,173
Amortized Cost	35,859		33,514
Fair Value	32,449		29,154
Subordinated debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	9,246		8,907
Amortized Cost	8,035		7,866
Fair Value	8,444		7,945
Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amortized Cost	266,800		261,879
Fair Value	$ 301,061		$ 297,430

[1] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See “Note 6. Fair Value Measurements”.

[2] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See “Note 6. Fair Value Measurements”.