Investments - Fair Value, Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2024		Sep. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	$ 7,525,291		$ 5,372,788
Amortized Cost	7,965,888		5,593,836
Fair Value	7,867,514	[1]	5,516,613	[2]
Senior secured
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	572,956		518,721
Amortized Cost	574,913		527,449
Fair Value	567,375		503,985
One stop
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	6,907,436		4,812,987
Amortized Cost	6,863,356		4,763,128
Fair Value	6,734,334		4,678,099
Second lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	17,364		32,173
Amortized Cost	16,345		33,514
Fair Value	13,298		29,154
Subordinated debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	27,535		8,907
Amortized Cost	27,085		7,866
Fair Value	26,972		7,945
Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amortized Cost	484,189		261,879
Fair Value	$ 525,535		$ 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”.