Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities 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]
Investments, at fair value	$ 5,446,356	[1]	$ 4,894,886	[2]
Money market funds	37,208		38,317
Forward currency contracts	32,333		2,716
Total assets, at fair value:	5,515,897		4,935,919
Forward currency contracts			(2,626)
Total liabilities, at fair value:			(2,626)
Debt Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	5,168,537		4,709,148
Equity Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	277,819		185,738
Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Money market funds	37,208		38,317
Forward currency contracts	0		0
Total assets, at fair value:	37,208		38,825
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		508
Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Money market funds	0		0
Forward currency contracts	32,333		2,716
Total assets, at fair value:	32,333		2,716
Forward currency contracts			(2,626)
Total liabilities, at fair value:			(2,626)
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:	5,446,356		4,894,378
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	5,168,537		4,709,148
Level 3 | Equity Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	$ 277,819		$ 185,230

[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.