v3.26.1
Fair Value Measurements - Schedule of Changes in Investments Measured At Fair Value (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 $ 8,769,389 $ 8,235,293
Fundings of (proceeds from) revolving loans, net 14,147 7,592
Fundings of investments 367,296 1,781,515
PIK interest and non-cash dividends 66,581 63,530
Proceeds from non-cash dividends (358) (4,288)
Proceeds from principal payments and sales of portfolio investments (828,977) (1,145,922)
Fair value, end of period 8,196,353 8,961,549
Location, Statement of Income, Balance [Axis]: us-gaap:DerivativeAndForeignCurrencyTransactionPriceChangeUnrealizedGainLossOperatingBeforeTax    
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 (174,193) 25,871
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 1,040 (1,402)
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 (16,020) 35,666
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 12,688 5,524
Location, Statement of Income, Balance [Axis]: us-gaap:RealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax    
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 (15,240) (41,830)
Debt Investments    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning of period 8,123,107 7,656,873
Fundings of (proceeds from) revolving loans, net 14,147 7,592
Fundings of investments 342,499 1,717,810
PIK interest and non-cash dividends 49,157 40,439
Proceeds from non-cash dividends 0 0
Proceeds from principal payments and sales of portfolio investments (795,633) (1,129,187)
Fair value, end of period 7,612,642 8,331,702
Debt Investments | Location, Statement of Income, Balance [Axis]: us-gaap:DerivativeAndForeignCurrencyTransactionPriceChangeUnrealizedGainLossOperatingBeforeTax    
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 (97,354) 40,145
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 1,040 (1,402)
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 (16,003) 35,668
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 12,688 5,524
Debt Investments | Location, Statement of Income, Balance [Axis]: us-gaap:RealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax    
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 (21,006) (41,760)
Equity Investments    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning of period 646,282 578,420
Fundings of (proceeds from) revolving loans, net 0 0
Fundings of investments 24,797 63,705
PIK interest and non-cash dividends 17,424 23,091
Proceeds from non-cash dividends (358) (4,288)
Proceeds from principal payments and sales of portfolio investments (33,344) (16,735)
Fair value, end of period 583,711 629,847
Equity Investments | Location, Statement of Income, Balance [Axis]: us-gaap:DerivativeAndForeignCurrencyTransactionPriceChangeUnrealizedGainLossOperatingBeforeTax    
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 (76,839) (14,274)
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 (17) (2)
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
Equity Investments | Location, Statement of Income, Balance [Axis]: us-gaap:RealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax    
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 $ 5,766 $ (70)