Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2025		Sep. 30, 2024
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	$ 8,769,389	[1]	$ 8,235,411	[2]
Money market funds	93,656		213,101
Forward currency contracts	185		5,695
Interest rate swaps	20,408		31,486
Total assets, at fair value:	8,883,638		8,485,693
Forward currency contracts	(12,798)		(7,691)
Interest rate swaps	(689)
Total liabilities, at fair value:	(13,487)		(7,691)
Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Money market funds	93,656		213,101
Forward currency contracts	0		0
Interest rate swaps	0		0
Total assets, at fair value:	93,656		213,219
Forward currency contracts	0		0
Interest rate swaps	0
Total liabilities, 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	185		5,695
Interest rate swaps	20,408		31,486
Total assets, at fair value:	20,593		37,181
Forward currency contracts	(12,798)		(7,691)
Interest rate swaps	(689)
Total liabilities, at fair value:	(13,487)		(7,691)
Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Money market funds	0		0
Forward currency contracts	0		0
Interest rate swaps	0		0
Total assets, at fair value:	8,769,389		8,235,293
Forward currency contracts	0		0
Interest rate swaps	0
Total liabilities, at fair value:	0		0
Debt investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	8,123,107		7,656,873
Debt investments | Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	0		0
Debt investments | Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	0		0
Debt investments | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	8,123,107		7,656,873
Equity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	646,282		578,538
Equity investments | Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	0		118
Equity investments | Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	0		0
Equity investments | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments, at fair value	$ 646,282		$ 578,420

[1] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.

[2] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.