Portfolio Investments - Significant Unobservable Inputs for Level 3 Investments (Details) $ in Thousands	Jun. 30, 2024 USD ($)		Sep. 30, 2023 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 4,189,323	[1]	$ 1,927,237	[2]
Senior secured debt
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	3,686,271		1,817,981
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	2,752,013		1,089,140
Level 3 | Senior secured debt
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	2,511,476		1,063,761
Level 3 | Senior secured debt | Market Yield | Market Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 2,374,154		$ 906,639
Level 3 | Senior secured debt | Market Yield | Market Yield | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.090		0.100
Level 3 | Senior secured debt | Market Yield | Market Yield | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.390		0.320
Level 3 | Senior secured debt | Market Yield | Market Yield | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.115		0.133
Level 3 | Senior secured debt | Revenue Multiple | Market Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 13,614
Level 3 | Senior secured debt | Revenue Multiple | Market Yield | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.4
Level 3 | Senior secured debt | Revenue Multiple | Market Yield | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.6
Level 3 | Senior secured debt | Revenue Multiple | Market Yield | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.5
Level 3 | Senior secured debt | Transaction Price | Transaction Precedent
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 58,673		$ 93,170
Level 3 | Senior secured debt | Broker Quoted Price | Broker Quotations
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	65,035		63,952
Level 3 | Subordinated debt | Market Yield | Market Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 125,319		$ 13,792
Level 3 | Subordinated debt | Market Yield | Market Yield | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.100		0.090
Level 3 | Subordinated debt | Market Yield | Market Yield | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.130		0.110
Level 3 | Subordinated debt | Market Yield | Market Yield | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.119		0.100
Level 3 | Subordinated debt | Transaction Price | Market Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 98,000
Level 3 | Common equity and warrants & preferred equity | Revenue Multiple | Enterprise Value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 9,635		$ 3,424
Level 3 | Common equity and warrants & preferred equity | Revenue Multiple | Enterprise Value | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	5.8		0.7
Level 3 | Common equity and warrants & preferred equity | Revenue Multiple | Enterprise Value | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	6.8		3.2
Level 3 | Common equity and warrants & preferred equity | Revenue Multiple | Enterprise Value | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	6.3		0.7
Level 3 | Common equity and warrants & preferred equity | Transaction Price | Transaction Precedent
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value			$ 332
Level 3 | Common equity and warrants & preferred equity | EBITDA Multiple | Enterprise Value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 7,583		$ 7,831
Level 3 | Common equity and warrants & preferred equity | EBITDA Multiple | Enterprise Value | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.0		6.0
Level 3 | Common equity and warrants & preferred equity | EBITDA Multiple | Enterprise Value | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	15.1		15.1
Level 3 | Common equity and warrants & preferred equity | EBITDA Multiple | Enterprise Value | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	14.0		12.9

[1] Non-Control/Non-Affiliate Investments are investments that are neither Control Investments nor Affiliate Investments. Control Investments generally are defined by the Investment Company Act of 1940, as amended (the "Investment Company Act"), as investments in companies in which the Company owns more than 25% of the voting securities and/or has the power to exercise control over the management or policies of the company. Affiliate Investments generally are defined by the Investment Company Act as investments in companies in which the Company owns between 5% and 25% of the voting securities.

[2] Non-Control/Non-Affiliate Investments are investments that are neither Control Investments nor Affiliate Investments. Control Investments generally are defined by the Investment Company Act of 1940, as amended (the "Investment Company Act"), as investments in companies in which the Company owns more than 25% of the voting securities and/or has the power to exercise control over the management or policies of the company. Affiliate Investments generally are defined by the Investment Company Act as investments in companies in which the Company owns between 5% and 25% of the voting securities.