Investments - Fair Value, Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2024		Sep. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	$ 5,209,821		$ 5,372,788
Amortized Cost	5,443,610		5,593,836
Fair Value	5,394,737	[1]	5,516,613	[2]
Senior secured
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	441,276		518,721
Amortized Cost	446,527		527,449
Fair Value	434,472		503,985
One stop
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	4,730,094		4,812,987
Amortized Cost	4,681,247		4,763,128
Fair Value	4,602,923		4,678,099
Second lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	28,777		32,173
Amortized Cost	29,309		33,514
Fair Value	25,486		29,154
Subordinated debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	9,674		8,907
Amortized Cost	9,066		7,866
Fair Value	9,024		7,945
Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amortized Cost	277,461		261,879
Fair Value	$ 322,832		$ 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”.