Fair Value Measurements - Fair Value Measurement Inputs and Valuation Techniques (Details) $ in Thousands	Jun. 30, 2024 USD ($)		Sep. 30, 2023 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 7,867,514	[1]	$ 5,516,613	[2]
Level 3 | Senior secured loans | Yield analysis
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 567,375		$ 503,985
Level 3 | Senior secured loans | Yield analysis | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.075		0.080
Level 3 | Senior secured loans | Yield analysis | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.155		0.178
Level 3 | Senior secured loans | Yield analysis | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.105		0.106
Level 3 | Senior secured loans | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 15,120		$ 44,510
Level 3 | Senior secured loans | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	2.1		5.2
Level 3 | Senior secured loans | Market comparable companies | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	33.3		24.0
Level 3 | Senior secured loans | Market comparable companies | Weighted Average | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	12.9		14.7
Level 3 | One stop loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 52,785
Level 3 | One stop loans | Yield analysis
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	6,681,549		$ 4,678,099
Level 3 | One stop loans | Yield analysis | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	5,557,365		3,895,527
Level 3 | One stop loans | Yield analysis | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 1,124,184		$ 782,572
Level 3 | One stop loans | Yield analysis | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.070		0.073
Level 3 | One stop loans | Yield analysis | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.193		0.263
Level 3 | One stop loans | Yield analysis | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.108		0.109
Level 3 | One stop loans | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 99,389		$ 123,841
Level 3 | One stop loans | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	2.8		4.0
Level 3 | One stop loans | Market comparable companies | Minimum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	1.0		1.5
Level 3 | One stop loans | Market comparable companies | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	39.0		34.0
Level 3 | One stop loans | Market comparable companies | Maximum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	18.4		27.0
Level 3 | One stop loans | Market comparable companies | Weighted Average | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	16.1		16.3
Level 3 | One stop loans | Market comparable companies | Weighted Average | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	8.3		7.7
Level 3 | Subordinated debt and second lien loans | Yield analysis
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 40,270		$ 37,099
Level 3 | Subordinated debt and second lien loans | Yield analysis | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.093		0.103
Level 3 | Subordinated debt and second lien loans | Yield analysis | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.158		0.245
Level 3 | Subordinated debt and second lien loans | Yield analysis | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.131		0.146
Level 3 | Subordinated debt and second lien loans | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	5.5		8.3
Level 3 | Subordinated debt and second lien loans | Market comparable companies | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	24.0		22.0
Level 3 | Subordinated debt and second lien loans | Market comparable companies | Weighted Average | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	16.4		19.5
Level 3 | Equity | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 525,442		$ 297,330
Level 3 | Equity | Market comparable companies | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	470,773		258,046
Level 3 | Equity | Market comparable companies | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 54,669		$ 39,284
Level 3 | Equity | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	5.5		5.2
Level 3 | Equity | Market comparable companies | Minimum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	1.3		1.5
Level 3 | Equity | Market comparable companies | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	35.0		35.0
Level 3 | Equity | Market comparable companies | Maximum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	18.4		18.0
Level 3 | Equity | Market comparable companies | Weighted Average | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	16.5		16.9
Level 3 | Equity | Market comparable companies | Weighted Average | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	10.4		9.6

[1] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See “Note 6. Fair Value Measurements”.

[2] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See “Note 6. Fair Value Measurements”.