Fair Value Measurements - Fair Value Measurement Inputs and Valuation Techniques (Details) $ in Thousands	Mar. 31, 2023 USD ($)		Sep. 30, 2022 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 5,486,352	[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	$ 480,307		$ 472,873
Level 3 | Senior secured | Market rate approach | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	0.075		0.068
Level 3 | Senior secured | Market rate approach | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	0.250		0.200
Level 3 | Senior secured | Market rate approach | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	0.108		0.096
Level 3 | Senior secured | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 29,958		$ 25,661
Level 3 | Senior secured | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	4.1		6.5
Level 3 | Senior secured | Market comparable companies | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, 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 owned, fair value, measurement input	15.0		14.7
Level 3 | Senior secured | Collateral analysis
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 283		$ 0
Level 3 | One stop | Market rate approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	4,708,204		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	3,963,336		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	$ 744,868		$ 659,117
Level 3 | One stop | Market rate approach | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	0.070		0.070
Level 3 | One stop | Market rate approach | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	0.223		0.173
Level 3 | One stop | Market rate approach | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	0.109		0.096
Level 3 | One stop | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 99,661		$ 60,948
Level 3 | One stop | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, 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 owned, fair value, measurement input	2.0		2.0
Level 3 | One stop | Market comparable companies | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, 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 owned, fair value, 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 owned, fair value, measurement input	16.1		16.4
Level 3 | One stop | Market comparable companies | Weighted Average | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, 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	$ 28,586		$ 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 owned, fair value, measurement input	0.098		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 owned, fair value, measurement input	0.153		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 owned, fair value, measurement input	0.135		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 owned, fair value, measurement input	6.5		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 owned, fair value, 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 owned, fair value, measurement input	20.8		21.0
Level 3 | Equity | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 268,972		$ 277,819
Level 3 | Equity | Market comparable companies | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	231,299		237,257
Level 3 | Equity | Market comparable companies | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 37,673		$ 40,562
Level 3 | Equity | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	4.1		4.5
Level 3 | Equity | Market comparable companies | Minimum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	2.0		2.0
Level 3 | Equity | Market comparable companies | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	37.0		38.0
Level 3 | Equity | Market comparable companies | Maximum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, 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 owned, fair value, measurement input	16.8		18.0
Level 3 | Equity | Market comparable companies | Weighted Average | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	10.2		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.