Portfolio Investments - Schedule of Roll-forward of the Changes in Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 5,316,963	$ 3,140,726
Purchases	613,861	704,347
Sales and repayments	(180,659)	(143,943)
Transfers in		37,213
Transfers out		(12,929)
Capitalized PIK interest income	2,059	2,695
Accretion of OID	6,755	6,520
Fair value, ending balance	5,753,856	3,710,062
Net unrealized appreciation (depreciation) relating to Level 3 assets still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations	$ (2,416)	$ (18,494)
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
Net unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	$ (5,384)	$ (22,803)
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
Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	$ 261	$ (1,764)
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
Senior Secured Debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 5,020,054	$ 2,890,580
Purchases	613,861	675,027
Sales and repayments	(171,157)	(143,757)
Transfers in		36,264
Transfers out		(12,929)
Capitalized PIK interest income	1,327	2,009
Accretion of OID	6,741	6,388
Fair value, ending balance	5,468,194	3,428,782
Net unrealized appreciation (depreciation) relating to Level 3 assets still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations	100	(18,727)
Senior Secured Debt | Net unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	(2,868)	(23,036)
Senior Secured Debt | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	236	(1,764)
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	265,289	223,942
Purchases	0	29,320
Sales and repayments	(9,502)	(186)
Transfers in		0
Transfers out		0
Capitalized PIK interest income	508	686
Accretion of OID	14	132
Fair value, ending balance	256,449	255,274
Net unrealized appreciation (depreciation) relating to Level 3 assets still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations	115	1,380
Subordinated Debt (including CLO Notes and Credit-Linked Notes) | Net unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	115	1,380
Subordinated Debt (including CLO Notes and Credit-Linked Notes) | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	25	0
Preferred Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	18,658	14,008
Purchases	0	0
Sales and repayments	0	0
Transfers in		949
Transfers out		0
Capitalized PIK interest income	224	0
Accretion of OID	0	0
Fair value, ending balance	18,935	15,629
Net unrealized appreciation (depreciation) relating to Level 3 assets still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations	53	672
Preferred Equity | Net unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	53	672
Preferred Equity | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	0	0
Common Equity and Warrants
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	12,962	12,196
Purchases	0	0
Sales and repayments	0	0
Transfers in		0
Transfers out		0
Capitalized PIK interest income	0	0
Accretion of OID	0	0
Fair value, ending balance	10,278	10,377
Net unrealized appreciation (depreciation) relating to Level 3 assets still held and reported within net unrealized appreciation (depreciation) in the Consolidated Statement of Operations	(2,684)	(1,819)
Common Equity and Warrants | Net unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	(2,684)	(1,819)
Common Equity and Warrants | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net unrealized appreciation (depreciation)/Net realized gains (losses)	$ 0	$ 0