v3.25.0.1
Fair Value Measurements - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($)
$ in Thousands
 3 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning of period $ 8,235,293 $ 5,516,513
Fundings of (proceeds from) revolving loans, net 2,169 235
Fundings of investments 909,514 73,242
PIK interest and non-cash dividends 20,475 17,966
Proceeds from non-cash dividends (1,824)  
Proceeds from principal payments and sales of portfolio investments (450,174) (179,411)
Fair value, end of period 8,685,129 5,443,322
Net change in unrealized appreciation (depreciation) on investments    
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 $ 43,629 $ (9,304)
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Net change in unrealized appreciation (depreciation) on investment transactions Net change in unrealized appreciation (depreciation) on investment transactions
Net translation of investments in foreign currencies    
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 $ (46,444) $ 19,498
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Translation of assets and liabilities in foreign currencies Translation of assets and liabilities in foreign currencies
Realized gain (loss) on investments    
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 $ (25,356) $ 909
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Net realized gain (loss) on investment transactions Net realized gain (loss) on investment transactions
Realized gain (loss) on translation of investments in foreign currencies    
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 $ (2,722) $ 1
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Foreign currency transactions Foreign currency transactions
Accretion of discounts and amortization of premiums    
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 $ 569 $ 3,673
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Gross Investment Income, Operating Gross Investment Income, Operating
Debt Investments    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning of period $ 7,656,873 $ 5,219,183
Fundings of (proceeds from) revolving loans, net 2,169 235
Fundings of investments 871,564 70,461
PIK interest and non-cash dividends 13,211 13,715
Proceeds from non-cash dividends 0  
Proceeds from principal payments and sales of portfolio investments (442,071) (176,449)
Fair value, end of period 8,076,256 5,142,366
Debt Investments | Net change in unrealized appreciation (depreciation) on investments    
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 48,673 (8,002)
Debt Investments | Net translation of investments in foreign currencies    
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 (46,421) 19,491
Debt Investments | Realized gain (loss) on investments    
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 (25,589) 58
Debt Investments | Realized gain (loss) on translation of investments in foreign currencies    
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 (2,722) 1
Debt Investments | Accretion of discounts and amortization of premiums    
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 569 3,673
Equity Investments    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value, beginning of period 578,420 297,330
Fundings of (proceeds from) revolving loans, net 0 0
Fundings of investments 37,950 2,781
PIK interest and non-cash dividends 7,264 4,251
Proceeds from non-cash dividends (1,824)  
Proceeds from principal payments and sales of portfolio investments (8,103) (2,962)
Fair value, end of period 608,873 300,956
Equity Investments | Net change in unrealized appreciation (depreciation) on investments    
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 (5,044) (1,302)
Equity Investments | Net translation of investments in foreign currencies    
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 (23) 7
Equity Investments | Realized gain (loss) on investments    
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 233 851
Equity Investments | Realized gain (loss) on translation of investments in foreign currencies    
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 | Accretion of discounts and amortization of premiums    
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