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 $ 1,087,964 $ 1,152,369
Net realized gain (loss) 70,552 (30,734)
Net change in unrealized appreciation (depreciation) (67,414) 39,674
Purchases, PIK interest, net discount accretion and non-cash exchanges 308,159 554,128
Sales, repayments and non-cash exchanges (531,928) (749,017)
Transfers in/out of Level 3 0 0
Ending balance 867,333 966,420
Net change in unrealized appreciation reported within the net change in unrealized appreciation on investments in our consolidated statements operations attributable to our Level 3 assets still held at the reporting date (7,866) 22,441
Debt Investments    
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Beginning balance 801,754 916,796
Net realized gain (loss) (9,546) (30,009)
Net change in unrealized appreciation (depreciation) (8,473) 4,579
Purchases, PIK interest, net discount accretion and non-cash exchanges 300,483 543,409
Sales, repayments and non-cash exchanges (435,684) (746,131)
Transfers in/out of Level 3 0 0
Ending balance 648,534 688,644
Net change in unrealized appreciation reported within the net change in unrealized appreciation on investments in our consolidated statements operations attributable to our Level 3 assets still held at the reporting date (17,686) (11,620)
Equity Investments    
Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Beginning balance 286,210 235,573
Net realized gain (loss) 80,098 (725)
Net change in unrealized appreciation (depreciation) (58,941) 35,095
Purchases, PIK interest, net discount accretion and non-cash exchanges 7,676 10,719
Sales, repayments and non-cash exchanges (96,244) (2,886)
Transfers in/out of Level 3 0 0
Ending balance 218,799 277,776
Net change in unrealized appreciation reported within the net change in unrealized appreciation on investments in our consolidated statements operations attributable to our Level 3 assets still held at the reporting date $ 9,820 $ 34,061