Fair Value Measurements - Fair Value Measurement Inputs and Valuation Techniques (Details) $ in Thousands	Jun. 30, 2023 USD ($)		Sep. 30, 2022 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 5,525,009	[1]	$ 5,446,356	[2]
Level 3 | Senior secured | Market rate approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 488,475		$ 472,873
Level 3 | Senior secured | Market rate approach | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.078		0.068
Level 3 | Senior secured | Market rate approach | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.198		0.200
Level 3 | Senior secured | Market rate approach | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.100		0.096
Level 3 | Senior secured | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 43,603		$ 25,661
Level 3 | Senior secured | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	4.6		6.5
Level 3 | Senior secured | Market comparable companies | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	26.0		26.2
Level 3 | Senior secured | Market comparable companies | Weighted Average | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	15.1		14.7
Level 3 | Senior secured | Collateral analysis
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value			$ 0
Level 3 | One stop | Market rate approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 4,719,262		4,668,609
Level 3 | One stop | Market rate approach | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	3,947,861		4,009,492
Level 3 | One stop | Market rate approach | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 771,401		$ 659,117
Level 3 | One stop | Market rate approach | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.068		0.070
Level 3 | One stop | Market rate approach | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.220		0.173
Level 3 | One stop | Market rate approach | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.105		0.096
Level 3 | One stop | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 92,707		$ 60,948
Level 3 | One stop | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	4.0		4.5
Level 3 | One stop | Market comparable companies | Minimum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	1.6		2.0
Level 3 | One stop | Market comparable companies | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	33.0		37.3
Level 3 | One stop | Market comparable companies | Maximum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	20.0		22.0
Level 3 | One stop | Market comparable companies | Weighted Average | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	16.3		16.4
Level 3 | One stop | Market comparable companies | Weighted Average | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	7.6		8.3
Level 3 | Subordinated debt and second lien loans | Market rate approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 33,109		$ 27,055
Level 3 | Subordinated debt and second lien loans | Market rate approach | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.123		0.098
Level 3 | Subordinated debt and second lien loans | Market rate approach | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.228		0.138
Level 3 | Subordinated debt and second lien loans | Market rate approach | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.137		0.122
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	6.7		6.5
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	22.0		23.0
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	18.9		21.0
Level 3 | Equity | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 284,054		$ 277,819
Level 3 | Equity | Market comparable companies | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	245,430		237,257
Level 3 | Equity | Market comparable companies | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 38,624		$ 40,562
Level 3 | Equity | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	4.6		4.5
Level 3 | Equity | Market comparable companies | Minimum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	2.5		2.0
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		38.0
Level 3 | Equity | Market comparable companies | Maximum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	20.0		24.4
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	16.7		18.0
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	9.9		12.6

[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.