Fair Value Measurements - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 5,516,513	$ 5,446,356
Fundings of (proceeds from) revolving loans, net	4,658	2,449
Fundings of investments	554,521	583,733
PIK interest and non-cash dividends	50,441	38,427
Proceeds from non-cash dividends	(47)
Proceeds from principal payments and sales of portfolio investments	(867,233)	(550,843)
Fair value, end of period	7,867,421	5,524,900
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	$ (35,597)	$ (5,083)
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	$ 14,455	$ 47,824
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	$ (49,505)	$ (46,555)
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	$ (6,831)	$ (1,402)
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	$ 10,996	$ 9,994
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	$ 5,219,183	$ 5,168,537
Fundings of (proceeds from) revolving loans, net	4,658	2,449
Fundings of investments	484,277	556,489
PIK interest and non-cash dividends	36,551	27,660
Proceeds from non-cash dividends	0
Proceeds from principal payments and sales of portfolio investments	(855,953)	(534,196)
Fair value, end of period	7,341,979	5,240,846
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	(41,400)	11,678
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	14,457	47,721
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	(51,729)	(48,084)
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	(6,831)	(1,402)
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	10,996	9,994
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	297,330	277,819
Fundings of (proceeds from) revolving loans, net	0	0
Fundings of investments	70,244	27,244
PIK interest and non-cash dividends	13,890	10,767
Proceeds from non-cash dividends	(47)
Proceeds from principal payments and sales of portfolio investments	(11,280)	(16,647)
Fair value, end of period	525,442	284,054
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	5,803	(16,761)
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)	103
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	2,224	1,529
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