Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities 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]
Investments, at fair value	$ 5,486,352	[1]	$ 5,446,356	[2]
Money market funds	64,263		37,208
Forward currency contracts	15,745		32,333
Total assets, at fair value:	5,566,360		5,515,897
Forward currency contracts	(307)
Total liabilities, at fair value:	(307)
Debt Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	5,217,380		5,168,537
Equity Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	268,972		277,819
Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Money market funds	64,263		37,208
Forward currency contracts	0		0
Total assets, at fair value:	64,263		37,208
Forward currency contracts	0
Total liabilities, at fair value:	0
Level 1 | Debt Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	0		0
Level 1 | Equity Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	0		0
Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Money market funds	0		0
Forward currency contracts	15,745		32,333
Total assets, at fair value:	15,745		32,333
Forward currency contracts	(307)
Total liabilities, at fair value:	(307)
Level 2 | Debt Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	0		0
Level 2 | Equity Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	0		0
Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Money market funds	0		0
Forward currency contracts	0		0
Total assets, at fair value:	5,486,352		5,446,356
Forward currency contracts	0
Total liabilities, at fair value:	0
Level 3 | Debt Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	5,217,380		5,168,537
Level 3 | Equity Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at 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.