Investments - Fair Value, Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2022		Sep. 30, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	$ 5,350,458		$ 4,795,665
Amortized Cost	5,569,604		4,895,397
Fair Value	5,446,356	[1]	4,894,886	[2]
Senior secured
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	519,188		816,316
Amortized Cost	518,216		803,520
Fair Value	472,873		784,805
One stop
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	4,801,600		3,936,606
Amortized Cost	4,786,118		3,913,331
Fair Value	4,668,609		3,882,314
Second lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	25,801		42,571
Amortized Cost	29,337		41,946
Fair Value	23,240		41,857
Subordinated debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	3,869		172
Amortized Cost	3,814		171
Fair Value	3,815		172
Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amortized Cost	232,119		136,429
Fair Value	$ 277,819		$ 185,738

[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 substantially all investments were valued using significant unobservable inputs. See Note 6. Fair Value Measurements.