v3.25.2
Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2025
Sep. 30, 2024
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments, at fair value $ 8,961,549 [1] $ 8,235,411 [2]
Money market funds 108,869 213,101
Forward currency contracts 288 5,695
Interest rate swaps 19,859 31,486
Total assets, at fair value: 9,090,565 8,485,693
Forward currency contracts (21,132) (7,691)
Total liabilities, at fair value: (21,132) (7,691)
Level 1    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Money market funds 108,869 213,101
Forward currency contracts 0 0
Interest rate swaps 0 0
Total assets, at fair value: 108,869 213,219
Forward currency contracts 0 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 288 5,695
Interest rate swaps 19,859 31,486
Total assets, at fair value: 20,147 37,181
Forward currency contracts (21,132) (7,691)
Total liabilities, at fair value: (21,132) (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: 8,961,549 8,235,293
Forward currency contracts 0 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 8,331,702 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 8,331,702 7,656,873
Equity investments    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Investments, at fair value 629,847 578,538
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 $ 629,847 $ 578,420
[1] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6.
[2] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6.