v3.25.2
Fair Value Measurements - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($)
$ in Thousands
 9 Months Ended
Jun. 30, 2025
Jun. 30, 2024
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 7,592 4,658
Fundings of investments 1,781,515 554,521
PIK interest and non-cash dividends 63,530 50,441
Proceeds from non-cash dividends (4,288) (47)
Proceeds from principal payments and sales of portfolio investments (1,145,922) (867,233)
Fair value, end of period 8,961,549 7,867,421
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 $ 25,871 $ (35,597)
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 $ 35,666 $ 14,455
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 $ (41,830) $ (49,505)
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 $ (1,402) $ (6,831)
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 $ 5,524 $ 10,996
Transfer in - GBDC 3 Merger   $ 2,675,050
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 7,592 4,658
Fundings of investments 1,717,810 484,277
PIK interest and non-cash dividends 40,439 36,551
Proceeds from non-cash dividends 0 0
Proceeds from principal payments and sales of portfolio investments (1,129,187) (855,953)
Fair value, end of period 8,331,702 7,341,979
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 40,145 (41,400)
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 35,668 14,457
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 (41,760) (51,729)
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 (1,402) (6,831)
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 5,524 10,996
Transfer in - GBDC 3 Merger   2,527,770
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 63,705 70,244
PIK interest and non-cash dividends 23,091 13,890
Proceeds from non-cash dividends (4,288) (47)
Proceeds from principal payments and sales of portfolio investments (16,735) (11,280)
Fair value, end of period 629,847 525,442
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 (14,274) 5,803
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 (2) (2)
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 (70) 2,224
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
Transfer in - GBDC 3 Merger   $ 147,280