Fair Value Measurements - Fair Value Measurement Inputs and Valuation Techniques (Details) $ in Thousands	Sep. 30, 2025 USD ($)		Sep. 30, 2024 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 8,769,389	[1]	$ 8,235,411	[2]
Level 3 | Senior secured loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	442,477		502,386
Level 3 | Senior secured loans | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 67		$ 2,807
Level 3 | Senior secured loans | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.073		0.065
Level 3 | Senior secured loans | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	3.5		1.5
Level 3 | Senior secured loans | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.170		0.203
Level 3 | Senior secured loans | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	26.3		24.0
Level 3 | Senior secured loans | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.091		0.105
Level 3 | Senior secured loans | Weighted Average | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	14.2		12.3
Level 3 | One stop loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value			$ 7,057,577
Level 3 | One stop loans | Yield analysis
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 7,603,780
Level 3 | One stop loans | Broker/dealer bids or quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	12,029		52,681
Level 3 | One stop loans | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	46,465		64,786
Level 3 | One stop loans | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	6,600,901		6,036,312
Level 3 | One stop loans | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 1,002,879		$ 1,021,265
Level 3 | One stop loans | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.038		0.043
Level 3 | One stop loans | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	3.9		2.8
Level 3 | One stop loans | Minimum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	1.4		1.0
Level 3 | One stop loans | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.205		0.210
Level 3 | One stop loans | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	39.2		53.2
Level 3 | One stop loans | Maximum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	15.0		18.0
Level 3 | One stop loans | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.091		0.099
Level 3 | One stop loans | Weighted Average | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	15.7		16.0
Level 3 | One stop loans | Weighted Average | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	8.0		7.9
Level 3 | Subordinated debt and second lien loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 64,821		$ 44,229
Level 3 | Subordinated debt and second lien loans | Market comparable companies
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 132		$ 435
Level 3 | Subordinated debt and second lien loans | Minimum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.088		0.100
Level 3 | Subordinated debt and second lien loans | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	4.0		5.5
Level 3 | Subordinated debt and second lien loans | Maximum | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.220		0.155
Level 3 | Subordinated debt and second lien loans | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	24.0		24.0
Level 3 | Subordinated debt and second lien loans | Weighted Average | Market interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	0.111		0.133
Level 3 | Subordinated debt and second lien loans | Weighted Average | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	15.5		16.0
Level 3 | Equity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 646,282		$ 578,420
Level 3 | Equity | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	563,467		505,597
Level 3 | Equity | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 82,815		$ 72,823
Level 3 | Equity | Minimum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	4.0		5.5
Level 3 | Equity | Minimum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	1.4		1.2
Level 3 | Equity | Maximum | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	39.2		53.2
Level 3 | Equity | Maximum | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	21.3		18.0
Level 3 | Equity | Weighted Average | EBITDA multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	16.4		18.2
Level 3 | Equity | Weighted Average | Revenue multiples
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment company, investment owned, measurement input	8.8		8.1

[1] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.

[2] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.