v3.26.1
Fair Value of Financial Instruments - Reconciliation of Investments Measured at Fair Value Using Significant Unobservable Inputs Level 3 (Details) - Level 3 - USD ($)
$ in Thousands
 9 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 2,729,010 $ 1,917,839
Net realized gain (loss) $ (31,315) $ 8,355
Net change in unrealized depreciation/appreciation Gain (Loss) on Investments Gain (Loss) on Investments
Net change in unrealized appreciation (depreciation) $ 45,354 $ (14,003)
Purchases, PIK interest, net discount accretion and non-cash exchanges 617,331 1,099,094
Sales, repayments and non-cash exchanges (908,093) (669,456)
Ending Balance 2,424,209 2,341,829
Net change in unrealized depreciation reported within the net change in unrealized depreciation on investments in our consolidated statements of operations attributable to our Level 3 assets still held at the reporting date. $ (45,433) $ (6,247)
Net change in unrealized appreciation (depreciation) reported Net Change in Unrealized Appreciation (Depreciation) Net Change in Unrealized Appreciation (Depreciation)
First lien    
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 2,513,631 $ 1,746,697
Net realized gain (loss) (8,377) (12,997)
Net change in unrealized appreciation (depreciation) (27,499) (4,757)
Purchases, PIK interest, net discount accretion and non-cash exchanges 603,445 1,077,943
Sales, repayments and non-cash exchanges (850,504) (656,285)
Ending Balance 2,230,696 2,150,601
Net change in unrealized depreciation reported within the net change in unrealized depreciation on investments in our consolidated statements of operations attributable to our Level 3 assets still held at the reporting date. (31,919) (3,778)
Second Lien, Subordinated Debt and Equity Investments    
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Beginning Balance 215,379 171,142
Net realized gain (loss) 39,692 21,352
Net change in unrealized appreciation (depreciation) (17,855) (9,246)
Purchases, PIK interest, net discount accretion and non-cash exchanges 13,886 21,151
Sales, repayments and non-cash exchanges (57,589) (13,171)
Ending Balance 193,513 191,228
Net change in unrealized depreciation reported within the net change in unrealized depreciation on investments in our consolidated statements of operations attributable to our Level 3 assets still held at the reporting date. $ 13,514 $ 10,025