Fair Value Measurements - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 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	6,253	2,596
Fundings of investments	1,952,573	1,299,933
PIK interest and non-cash dividends	86,097	73,815
Proceeds from non-cash dividends	(6,436)	(47)
Proceeds from principal payments and sales of portfolio investments	(1,516,678)	(1,262,884)
Fair value, end of period	8,769,389	8,235,293
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	$ 50,582	$ (32,055)
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	$ 26,377	$ 41,757
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	$ (72,190)	$ (81,577)
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	$ (1,388)	$ (8,211)
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	$ 8,906	$ 10,403
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	6,253	2,596
Fundings of investments	1,860,091	1,186,368
PIK interest and non-cash dividends	55,390	52,552
Proceeds from non-cash dividends	0	0
Proceeds from principal payments and sales of portfolio investments	(1,480,752)	(1,250,895)
Fair value, end of period	8,123,107	7,656,873
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	63,763	(41,723)
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	26,391	41,755
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	(72,430)	(82,925)
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	(1,378)	(8,211)
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	8,906	10,403
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	92,482	113,565
PIK interest and non-cash dividends	30,707	21,263
Proceeds from non-cash dividends	(6,436)	(47)
Proceeds from principal payments and sales of portfolio investments	(35,926)	(11,989)
Fair value, end of period	646,282	578,420
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	(13,181)	9,668
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	(14)	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	240	1,348
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	(10)	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