Investments - Fair Value, Assets 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]
Principal	$ 8,253,915		$ 7,830,886
Amortized Cost	8,759,938		8,302,913
Fair Value	8,769,389	[1]	8,235,411	[2]
Senior secured
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	448,400		510,503
Amortized Cost	444,061		506,178
Fair Value	442,477		502,386
One stop
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	7,735,754		7,271,543
Amortized Cost	7,633,999		7,216,368
Fair Value	7,615,809		7,110,258
Second lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	31,317		18,430
Amortized Cost	29,607		17,125
Fair Value	26,409		14,054
Subordinated debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal	38,444		30,410
Amortized Cost	38,027		29,943
Fair Value	38,412		30,175
Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amortized Cost	614,244		533,299
Fair Value	$ 646,282		$ 578,538

[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.