Investments - Fair Value, Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2023		Sep. 30, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	$ 5,438,639		$ 5,350,458
Amortized Cost	5,618,695		5,569,604
Fair Value	5,486,352	[1]	5,446,356	[2]
Senior secured
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	522,766		519,188
Amortized Cost	510,454		518,216
Fair Value	480,590		472,873
One stop
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	4,883,738		4,801,600
Amortized Cost	4,831,531		4,786,118
Fair Value	4,708,204		4,668,609
Second lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	27,870		25,801
Amortized Cost	29,392		29,337
Fair Value	24,436		23,240
Subordinated debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	4,265		3,869
Amortized Cost	4,213		3,814
Fair Value	4,150		3,815
Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amortized Cost	243,105		232,119
Fair Value	$ 268,972		$ 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.