Portfolio Investments - Schedule of Financial Instruments Carried at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2025		Sep. 30, 2025
Schedule of Investments [Line Items]
Total investments at fair value	$ 7,433,861	[1]	$ 6,899,031	[2]
Cash equivalents	157,177		210,587
Derivative assets	19,544		24,182
Total assets at fair value	7,610,582		7,133,800
Derivative liabilities	1,189		0
Total liabilities at fair value	1,189
Senior secured debt
Schedule of Investments [Line Items]
Total investments at fair value	6,997,991		6,438,079
Subordinated debt (including CLO notes and credit linked notes)
Schedule of Investments [Line Items]
Total investments at fair value	406,637		429,299
Common equity and warrants
Schedule of Investments [Line Items]
Total investments at fair value	10,298		12,995
Preferred equity
Schedule of Investments [Line Items]
Total investments at fair value	18,935		18,658
Level 1
Schedule of Investments [Line Items]
Total investments at fair value	20		33
Cash equivalents	157,177		210,587
Derivative assets	0		0
Total assets at fair value	157,197		210,620
Derivative liabilities	0
Total liabilities at fair value	0
Level 1 | Senior secured debt
Schedule of Investments [Line Items]
Total investments at fair value	0		0
Level 1 | Subordinated debt (including CLO notes and credit linked notes)
Schedule of Investments [Line Items]
Total investments at fair value	0		0
Level 1 | Common equity and warrants
Schedule of Investments [Line Items]
Total investments at fair value	20		33
Level 1 | Preferred equity
Schedule of Investments [Line Items]
Total investments at fair value	0		0
Level 2
Schedule of Investments [Line Items]
Total investments at fair value	1,679,985		1,582,035
Cash equivalents	0		0
Derivative assets	19,544		24,182
Total assets at fair value	1,699,529		1,606,217
Derivative liabilities	1,189
Total liabilities at fair value	1,189
Level 2 | Senior secured debt
Schedule of Investments [Line Items]
Total investments at fair value	1,529,797		1,418,025
Level 2 | Subordinated debt (including CLO notes and credit linked notes)
Schedule of Investments [Line Items]
Total investments at fair value	150,188		164,010
Level 2 | Common equity and warrants
Schedule of Investments [Line Items]
Total investments at fair value	0		0
Level 2 | Preferred equity
Schedule of Investments [Line Items]
Total investments at fair value	0		0
Level 3
Schedule of Investments [Line Items]
Total investments at fair value	5,753,856		5,316,963
Cash equivalents	0		0
Derivative assets	0		0
Total assets at fair value	5,753,856		5,316,963
Derivative liabilities	0
Total liabilities at fair value	0
Level 3 | Senior secured debt
Schedule of Investments [Line Items]
Total investments at fair value	5,468,194		5,020,054
Level 3 | Subordinated debt (including CLO notes and credit linked notes)
Schedule of Investments [Line Items]
Total investments at fair value	256,449		265,289
Level 3 | Common equity and warrants
Schedule of Investments [Line Items]
Total investments at fair value	10,278		12,962
Level 3 | Preferred equity
Schedule of Investments [Line Items]
Total investments at fair value	$ 18,935		$ 18,658

[1] Non-Control/Non-Affiliate Investments are investments that are neither Control Investments nor Affiliate Investments. Control Investments generally are defined by the Investment Company Act of 1940, as amended (the “Investment Company Act”), as investments in companies in which the Company owns more than 25% of the voting securities and/or has the power to exercise control over the management or policies of the company. Affiliate Investments generally are defined by the Investment Company Act as investments in companies in which the Company owns between 5% and 25% of the voting securities.

[2] Non-Control/Non-Affiliate Investments are investments that are neither Control Investments nor Affiliate Investments. Control Investments generally are defined by the Investment Company Act as investments in companies in which the Company owns more than 25% of the voting securities and/or has the power to exercise control over the management or policies of the company. Affiliate Investments generally are defined by the Investment Company Act as investments in companies in which the Company owns between 5% and 25% of the voting securities.