Fair Value Measurements - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 5,446,356	$ 4,894,378
Fundings of (proceeds from) revolving loans, net	381	1,812
Fundings of investments	675,228	1,883,080
PIK interest and non-cash dividends	53,745	21,506
Proceeds from non-cash dividends	(10)
Proceeds from principal payments and sales of portfolio investments	(670,188)	(1,260,688)
Fair value, end of period	5,516,513	5,446,356
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	$ 14,278	$ (56,743)
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	$ 31,760	$ (65,930)
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	$ (46,496)	$ 19,270
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	$ (1,419)	$ 624
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	$ 12,878	$ 9,047
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	$ 5,168,537	$ 4,709,148
Fundings of (proceeds from) revolving loans, net	381	1,812
Fundings of investments	644,630	1,766,974
PIK interest and non-cash dividends	38,844	21,506
Proceeds from non-cash dividends	0
Proceeds from principal payments and sales of portfolio investments	(652,759)	(1,220,593)
Fair value, end of period	5,219,183	5,168,537
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	24,509	(53,378)
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	31,666	(65,751)
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	(48,084)	(852)
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	(1,419)	624
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	12,878	9,047
Equity Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	277,819	185,230
Fundings of (proceeds from) revolving loans, net	0	0
Fundings of investments	30,598	116,106
PIK interest and non-cash dividends	14,901	0
Proceeds from non-cash dividends	(10)
Proceeds from principal payments and sales of portfolio investments	(17,429)	(40,095)
Fair value, end of period	297,330	277,819
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	(10,231)	(3,365)
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	94	(179)
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	1,588	20,122
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