Fair Value Measurements - Fair Value Measurement Inputs and Valuation Techniques (Details) $ in Thousands	Sep. 30, 2022 USD ($)		Sep. 30, 2021 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 5,446,356	[1]	$ 4,894,886	[2]
Level 3 | Senior secured | Market rate approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 472,873		$ 778,413
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	0.068		0.025
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	0.200		0.148
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	0.096		0.056
Level 3 | Senior secured | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 25,661		$ 23,989
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	6.5		6.0
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	26.2		24.2
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	14.7		15.1
Level 3 | Senior secured | Market comparable
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value			$ 6,172
Level 3 | Senior secured | Collateral analysis
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value			$ 220
Level 3 | Senior secured | Collateral analysis | Minimum | Recovery rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input			0.016
Level 3 | One stop | Market rate approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 4,668,609		$ 3,882,314
Level 3 | One stop | Market rate approach | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	4,009,492		3,354,556
Level 3 | One stop | Market rate approach | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 659,117		$ 527,758
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		0.010
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	0.173		0.180
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	0.096		0.075
Level 3 | One stop | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 60,948		$ 76,290
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	4.5		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	37.3		35.0
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	22.0		18.5
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	16.4		15.5
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	8.3		8.0
Level 3 | Subordinated debt and second lien loans | Market rate approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value			$ 42,029
Level 3 | Subordinated debt and second lien loans | Market rate approach | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value			42,020
Level 3 | Subordinated debt and second lien loans | Market rate approach | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value			$ 9
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		0.068
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	0.138		0.195
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	0.122		0.095
Level 3 | Subordinated debt and second lien loans | Market comparable companies | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input			3.4
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		6.0
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	23.0		23.6
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	21.0		17.2
Level 3 | Subordinated debt and second lien loans | Market comparable
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 27,055
Level 3 | Equity | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	277,819		$ 185,230
Level 3 | Equity | Market comparable companies | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	237,257		159,620
Level 3 | Equity | Market comparable companies | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 40,562		$ 25,610
Level 3 | Equity | Market comparable companies | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	4.5		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	38.0		26.0
Level 3 | Equity | Market comparable companies | Maximum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value, measurement input	24.4		25.0
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	18.0		17.4
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	12.6		12.3

[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 substantially all investments were valued using significant unobservable inputs. See Note 6. Fair Value Measurements.