Investments - Fair Value, Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2023		Sep. 30, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	$ 5,403,099		$ 5,350,458
Amortized Cost	5,605,521		5,569,604
Fair Value	5,525,009	[1]	5,446,356	[2]
Senior secured
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	500,907		519,188
Amortized Cost	505,598		518,216
Fair Value	488,475		472,873
One stop
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	4,865,033		4,801,600
Amortized Cost	4,807,677		4,786,118
Fair Value	4,719,262		4,668,609
Second lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	28,579		25,801
Amortized Cost	29,422		29,337
Fair Value	25,437		23,240
Subordinated debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	8,580		3,869
Amortized Cost	7,698		3,814
Fair Value	7,672		3,815
Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amortized Cost	255,126		232,119
Fair Value	$ 284,163		$ 277,819

[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.