Fair Value Measurements - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 4,894,378	$ 4,238,210
Funding of (proceeds from) revolving loans, net	1,812	(12,170)
Fundings of investments	1,883,080	2,081,683
PIK interest	21,506	16,092
Proceeds from principal payments and sales of portfolio investments	(1,260,688)	(1,593,478)
Fair value, end of period	5,446,356	4,894,378
Debt and Equity Securities, Unrealized Gain (Loss)
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	$ (56,743)	$ 160,111
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Debt and equity securities, unrealized gain (loss)	Debt and equity securities, unrealized gain (loss)
Debt and Equity Securities, Realized Gain (Loss)
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	$ 19,894	$ 13,324
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net realized (gain) loss on investments	Net realized (gain) loss on investments
Interest Income, Operating
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	$ 9,047	$ (9,394)
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Interest income	Interest income
Foreign Currency Transaction Gain (Loss), Unrealized
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	$ (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
Debt Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 4,709,148	$ 4,146,013
Funding of (proceeds from) revolving loans, net	1,812	(12,170)
Fundings of investments	1,766,974	2,010,541
PIK interest	21,506	16,092
Proceeds from principal payments and sales of portfolio investments	(1,220,593)	(1,553,176)
Fair value, end of period	5,168,537	4,709,148
Debt Investments | Debt and Equity Securities, Unrealized Gain (Loss)
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	$ (53,378)	$ 116,561
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Debt and equity securities, unrealized gain (loss)	Debt and equity securities, unrealized gain (loss)
Debt Investments | Debt and Equity Securities, Realized Gain (Loss)
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	$ (228)	$ (5,319)
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net realized (gain) loss on investments	Net realized (gain) loss on investments
Debt Investments | Interest Income, Operating
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	$ 9,047	$ (9,394)
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Interest income	Interest income
Debt Investments | Foreign Currency Transaction Gain (Loss), Unrealized
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	$ (65,751)
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
Equity Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 185,230	$ 92,197
Funding of (proceeds from) revolving loans, net	0	0
Fundings of investments	116,106	71,142
PIK interest	0	0
Proceeds from principal payments and sales of portfolio investments	(40,095)	(40,302)
Fair value, end of period	277,819	185,230
Equity Investments | Debt and Equity Securities, Unrealized Gain (Loss)
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	$ (3,365)	$ 43,550
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Debt and equity securities, unrealized gain (loss)	Debt and equity securities, unrealized gain (loss)
Equity Investments | Debt and Equity Securities, Realized Gain (Loss)
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	$ 20,122	$ 18,643
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net realized (gain) loss on investments	Net realized (gain) loss on investments
Equity Investments | Interest Income, Operating
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
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Interest income	Interest income
Equity Investments | Foreign Currency Transaction Gain (Loss), Unrealized
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	$ (179)
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