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Fair Value of Financial Instruments - Reconciliation of Investments Measured at Fair Value Using Significant Unobservable Inputs Level 3 (Details) - Level 3 - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 1,917,839 $ 1,006,948
Net realized losses $ 5,941 $ (222)
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Realized Gain (Loss), Investment and Derivative, Operating, after Tax Realized Gain (Loss), Investment and Derivative, Operating, after Tax
Net change in unrealized depreciation/appreciation $ 2,578 $ (20,642)
Purchases, PIK interest, net discount accretion and non-cash exchanges 1,734,207 1,409,650
Sales, repayments and non-cash exchanges (914,517) (519,623)
Ending Balance 2,729,010 1,917,839
Net change in unrealized appreciation reported within the net change in unrealized appreciation on investments in our Consolidated Statements of Operations attributable to our Level 3 assets still held at the reporting date. $ 10,266 $ 14,476
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Unrealized Gain (Loss), Investment and Derivative, Operating, before Tax Unrealized Gain (Loss), Investment and Derivative, Operating, before Tax
First lien    
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 1,746,697 $ 906,166
Net realized losses (13,939) (8,125)
Net change in unrealized depreciation/appreciation (641) 77
Purchases, PIK interest, net discount accretion and non-cash exchanges 1,700,654 1,320,051
Sales, repayments and non-cash exchanges (919,140) (471,318)
Ending Balance 2,513,631 1,746,697
Net change in unrealized appreciation reported within the net change in unrealized appreciation on investments in our Consolidated Statements of Operations attributable to our Level 3 assets still held at the reporting date. (4,077) (9,278)
Second Lien, Subordinated Debt and Equity Investments    
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Beginning Balance 171,142 100,782
Net realized losses 7,998 8,347
Net change in unrealized depreciation/appreciation 1,937 (20,719)
Purchases, PIK interest, net discount accretion and non-cash exchanges 33,553 89,599
Sales, repayments and non-cash exchanges 4,623 (48,305)
Ending Balance 215,379 171,142
Net change in unrealized appreciation reported within the net change in unrealized appreciation on investments in our Consolidated Statements of Operations attributable to our Level 3 assets still held at the reporting date. $ 14,343 $ 23,754