Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2024		Sep. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	$ 7,867,514	[1]	$ 5,516,613	[2]
Money market funds	372,879		72,822
Forward currency contracts	23,368		15,011
Interest rate swaps	22,365		14,941
Total assets, at fair value:	8,265,844		5,604,446
Total liabilities, at fair value:	(16,830)		(70)
Forward currency contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liabilities	(1,003)		(70)
Interest rate swaps
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Interest rate swaps	2,083
Derivative liabilities	(15,827)
Debt investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	7,341,979		5,219,183
Equity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	525,535		297,430
Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Money market funds	372,879		72,822
Forward currency contracts	0		0
Total assets, at fair value:	372,972		72,922
Total liabilities, at fair value:	0		0
Level 1 | Forward currency contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liabilities	0		0
Level 1 | Interest rate swaps
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Interest rate swaps	0
Derivative liabilities	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	93		100
Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Money market funds	0		0
Forward currency contracts	23,368		15,011
Total assets, at fair value:	25,451		15,011
Total liabilities, at fair value:	(16,830)		(70)
Level 2 | Forward currency contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liabilities	(1,003)		(70)
Level 2 | Interest rate swaps
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Interest rate swaps	2,083
Derivative liabilities	(15,827)
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:	7,867,421		5,516,513
Total liabilities, at fair value:	0		0
Level 3 | Forward currency contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liabilities	0		0
Level 3 | Interest rate swaps
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Interest rate swaps	0
Derivative liabilities	0
Level 3 | Debt investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	7,341,979		5,219,183
Level 3 | Equity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	$ 525,442		$ 297,330

[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”.