v3.26.1
Fair Value Measurements - Schedule of Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($)
$ in Thousands
9 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning of period $ 6,804,856 $ 2,649,583
Fundings of (proceeds from) revolving loans, net 45,572 8,896
Purchases and fundings of investments 2,412,114 2,912,571
PIK interest and non-cash dividends 38,194 15,577
Proceeds from non-cash dividends (255)  
Proceeds from principal payments and sales of portfolio investments (829,832) (280,015)
Transfers into Level 3 46,027 7,992
Transfers out of Level 3 (63,031) (22,872)
Fair value, end of period 8,339,916 5,350,935
Location, Statement of Income, Balance [Axis]: us-gaap:DebtAndEquitySecuritiesRealizedGainLoss    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings (4,712) 1
Location, Statement of Income, Balance [Axis]: us-gaap:DebtAndEquitySecuritiesUnrealizedGainLoss    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings (112,454) 13,978
Location, Statement of Income, Balance [Axis]: us-gaap:ForeignCurrencyTransactionGainLossRealized    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings 4,956 (23)
Location, Statement of Income, Balance [Axis]: us-gaap:ForeignCurrencyTransactionGainLossUnrealized    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings (14,854) 35,679
Location, Statement of Income, Balance [Axis]: us-gaap:GrossInvestmentIncomeOperating    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings 13,335 9,568
Debt Investments    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning of period 6,740,019 2,638,140
Fundings of (proceeds from) revolving loans, net 45,572 8,896
Purchases and fundings of investments 2,335,409 2,879,760
PIK interest and non-cash dividends 36,268 14,928
Proceeds from non-cash dividends 0  
Proceeds from principal payments and sales of portfolio investments (786,964) (280,015)
Transfers into Level 3 46,027 7,992
Transfers out of Level 3 (63,031) (22,872)
Fair value, end of period 8,240,249 5,304,873
Debt Investments | Location, Statement of Income, Balance [Axis]: us-gaap:DebtAndEquitySecuritiesRealizedGainLoss    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings (5,301) 1
Debt Investments | Location, Statement of Income, Balance [Axis]: us-gaap:DebtAndEquitySecuritiesUnrealizedGainLoss    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings (111,245) 12,819
Debt Investments | Location, Statement of Income, Balance [Axis]: us-gaap:ForeignCurrencyTransactionGainLossRealized    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings 4,956 (23)
Debt Investments | Location, Statement of Income, Balance [Axis]: us-gaap:ForeignCurrencyTransactionGainLossUnrealized    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings (14,796) 35,679
Debt Investments | Location, Statement of Income, Balance [Axis]: us-gaap:GrossInvestmentIncomeOperating    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings 13,335 9,568
Equity Investments    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning of period 64,837 11,443
Fundings of (proceeds from) revolving loans, net 0 0
Purchases and fundings of investments 76,705 32,811
PIK interest and non-cash dividends 1,926 649
Proceeds from non-cash dividends (255)  
Proceeds from principal payments and sales of portfolio investments (42,868) 0
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Fair value, end of period 99,667 46,062
Equity Investments | Location, Statement of Income, Balance [Axis]: us-gaap:DebtAndEquitySecuritiesRealizedGainLoss    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings 589 0
Equity Investments | Location, Statement of Income, Balance [Axis]: us-gaap:DebtAndEquitySecuritiesUnrealizedGainLoss    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings (1,209) 1,159
Equity Investments | Location, Statement of Income, Balance [Axis]: us-gaap:ForeignCurrencyTransactionGainLossRealized    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings 0 0
Equity Investments | Location, Statement of Income, Balance [Axis]: us-gaap:ForeignCurrencyTransactionGainLossUnrealized    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings (58) 0
Equity Investments | Location, Statement of Income, Balance [Axis]: us-gaap:GrossInvestmentIncomeOperating    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings $ 0 $ 0