Fair Value Measurements - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2023	Jun. 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	2,449	2,865
Fundings of investments	583,733	1,678,450
PIK interest and non-cash dividends	38,427	14,692
Proceeds from principal payments and sales of portfolio investments	(550,843)	(955,184)
Fair value, end of period	5,524,900	5,613,456
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	(5,083)	(7,865)
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	47,824	(33,862)
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	(46,555)	15,772
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)	676
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	9,994	3,534
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	2,449	2,865
Fundings of investments	556,489	1,567,934
PIK interest and non-cash dividends	27,660	14,692
Proceeds from principal payments and sales of portfolio investments	(534,196)	(922,865)
Fair value, end of period	5,240,846	5,334,266
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	11,678	(7,064)
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	47,721	(33,801)
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)	(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	(1,402)	676
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	9,994	3,534
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	27,244	110,516
PIK interest and non-cash dividends	10,767	0
Proceeds from principal payments and sales of portfolio investments	(16,647)	(32,319)
Fair value, end of period	284,054	279,190
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	(16,761)	(801)
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	103	(61)
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	1,529	16,625
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