Fair Value Measurements - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2023	Mar. 31, 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	1,970	1,002
Fundings of investments	412,649	1,267,348
PIK interest and non-cash dividends	24,299	8,597
Proceeds from principal payments and sales of portfolio investments	(393,997)	(783,967)
Fair value, end of period	5,486,352	5,426,012
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	(48,171)	31,076
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	39,076	(9,064)
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	(1,470)	14,176
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	(1,264)	718
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	6,904	1,748
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	1,970	1,002
Fundings of investments	396,692	1,181,552
PIK interest and non-cash dividends	17,255	8,597
Proceeds from principal payments and sales of portfolio investments	(379,988)	(755,126)
Fair value, end of period	5,217,380	5,159,833
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	(28,249)	22,118
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	38,988	(9,071)
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	(3,465)	(853)
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	(1,264)	718
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	6,904	1,748
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	15,957	85,796
PIK interest and non-cash dividends	7,044	0
Proceeds from principal payments and sales of portfolio investments	(14,009)	(28,841)
Fair value, end of period	268,972	266,179
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	(19,922)	8,958
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	88	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	1,995	15,029
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