Portfolio Investments - Schedule of Roll-forward of the Changes in Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 5,820,951	$ 4,178,869	$ 5,316,963	$ 3,140,726
Purchases	580,954	853,200	1,752,999	2,189,516
Sales and repayments	(374,540)	(130,216)	(949,010)	(440,750)
Transfers in		603	1,527	37,816
Transfers out		(32,423)	(36,451)	(68,859)
Capitalized PIK interest income	5,349	2,287	9,601	7,438
Accretion of OID	4,346	4,080	17,061	15,940
Fair value, ending balance	6,027,524	4,910,435	6,027,524	4,910,435
Net unrealized appreciation (depreciation) relating to Level 3 assets still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations	$ (9,051)	$ 32,786	$ (77,803)	$ 34,314
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax	Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax	Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax	Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax
Transfers out		$ 32,423	$ 36,451	$ 68,859
Income Statement Location [Axis]: us-gaap:DebtAndEquitySecuritiesRealizedGainLoss
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	$ 1,532	$ 3,153	$ (7,177)	$ 1,482
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Non-control/Non-affiliate investments	Non-control/Non-affiliate investments	Non-control/Non-affiliate investments	Non-control/Non-affiliate investments
Income Statement Location [Axis]: us-gaap:DebtAndEquitySecuritiesUnrealizedGainLoss
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	$ (11,068)	$ 30,882	$ (77,989)	$ 27,126
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Unrealized gain (loss) on investments	Unrealized gain (loss) on investments	Unrealized gain (loss) on investments	Unrealized gain (loss) on investments
Senior Secured Debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 5,431,778	$ 3,893,763	$ 5,020,054	$ 2,890,580
Purchases	509,219	767,295	1,543,243	2,074,529
Sales and repayments	(340,083)	(129,693)	(869,460)	(439,936)
Transfers in		0	0	36,264
Transfers out		(31,820)	(36,451)	(68,256)
Capitalized PIK interest income	2,014	1,504	4,329	5,198
Accretion of OID	4,297	3,950	16,983	15,549
Fair value, ending balance	5,598,692	4,535,534	5,598,692	4,535,534
Net unrealized appreciation (depreciation) relating to Level 3 assets still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations	(7,988)	29,553	(71,158)	26,990
Investment exchanged, cost			900
Transfers out		31,820	36,451	68,256
Senior Secured Debt | Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Transfers out			(34,900)
Transfers out			34,900
Senior Secured Debt | Income Statement Location [Axis]: us-gaap:DebtAndEquitySecuritiesRealizedGainLoss
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	1,995	3,153	(8,662)	1,571
Senior Secured Debt | Income Statement Location [Axis]: us-gaap:DebtAndEquitySecuritiesUnrealizedGainLoss
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	(10,528)	27,382	(71,344)	20,035
Subordinated Debt (including CLO Notes and Credit-Linked Notes)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	365,503	256,237	265,289	223,942
Purchases	71,735	85,708	209,756	114,790
Sales and repayments	(33,159)	(266)	(71,865)	(557)
Transfers in		0	0	0
Transfers out		0	0	0
Capitalized PIK interest income	3,284	783	4,902	2,240
Accretion of OID	49	130	78	391
Fair value, ending balance	406,971	342,885	406,971	342,885
Net unrealized appreciation (depreciation) relating to Level 3 assets still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations	(523)	293	(1,361)	2,079
Transfers out		0	0	0
Subordinated Debt (including CLO Notes and Credit-Linked Notes) | Income Statement Location [Axis]: us-gaap:DebtAndEquitySecuritiesRealizedGainLoss
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	82	0	172	0
Subordinated Debt (including CLO Notes and Credit-Linked Notes) | Income Statement Location [Axis]: us-gaap:DebtAndEquitySecuritiesUnrealizedGainLoss
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	(523)	293	(1,361)	2,079
Preferred Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	13,303	16,582	18,658	14,008
Purchases	0	162	0	162
Sales and repayments	(838)	0	(7,225)	0
Transfers in		603	930	1,552
Transfers out		0	0	0
Capitalized PIK interest income	51	0	370	0
Accretion of OID	0	0	0	0
Fair value, ending balance	12,933	18,731	12,933	18,731
Net unrealized appreciation (depreciation) relating to Level 3 assets still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations	419	1,384	(1,566)	3,009
Transfers out		0	0	0
Preferred Equity | Income Statement Location [Axis]: us-gaap:DebtAndEquitySecuritiesRealizedGainLoss
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	(92)	0	1,766	0
Preferred Equity | Income Statement Location [Axis]: us-gaap:DebtAndEquitySecuritiesUnrealizedGainLoss
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	509	1,384	(1,566)	3,009
Common Equity and Warrants
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	10,367	12,287	12,962	12,196
Purchases	0	35	0	35
Sales and repayments	(460)	(257)	(460)	(257)
Transfers in		0	597	0
Transfers out		(603)	0	(603)
Capitalized PIK interest income	0	0	0	0
Accretion of OID	0	0	0	0
Fair value, ending balance	8,928	13,285	8,928	13,285
Net unrealized appreciation (depreciation) relating to Level 3 assets still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations	(959)	1,556	(3,718)	2,236
Investment exchanged, cost			600
Transfers out		603	0	603
Common Equity and Warrants | Income Statement Location [Axis]: us-gaap:DebtAndEquitySecuritiesRealizedGainLoss
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	(453)	0	(453)	(89)
Common Equity and Warrants | Income Statement Location [Axis]: us-gaap:DebtAndEquitySecuritiesUnrealizedGainLoss
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	$ (526)	$ 1,823	$ (3,718)	$ 2,003