Portfolio Investments - Schedule of Significant Unobservable Inputs for Level 3 Investments (Details) $ in Thousands	9 Months Ended
	Jun. 30, 2026 USD ($) investment	Sep. 30, 2025 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 6,796,814	$ 6,899,031	[1]
Senior secured debt
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 6,280,474	6,438,079
Subordinated debt
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of debt investments, change in valuation technique | investment	2
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 6,027,524	5,316,963
Level 3 | Senior secured debt
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	5,598,692	5,020,054
Level 3 | Senior secured debt | Market Yield | Market Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 5,405,800	$ 4,896,387
Level 3 | Senior secured debt | Market Yield | Market Yield | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.060	0.060
Level 3 | Senior secured debt | Market Yield | Market Yield | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.350	0.260
Level 3 | Senior secured debt | Market Yield | Market Yield | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.101	0.093
Level 3 | Senior secured debt | Revenue Multiple | Enterprise Value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 5,404	$ 10,924
Level 3 | Senior secured debt | Revenue Multiple | Enterprise Value | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.9	2.1
Level 3 | Senior secured debt | Revenue Multiple | Enterprise Value | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.1	2.3
Level 3 | Senior secured debt | Revenue Multiple | Enterprise Value | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.0	2.2
Level 3 | Senior secured debt | Transaction Price | Transaction Precedent
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 59,059	$ 109,808
Level 3 | Senior secured debt | Broker Quoted Price | Broker Quotations
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	128,429
Level 3 | Senior secured debt | EBITDA Multiple | Enterprise Value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value		$ 2,935
Level 3 | Senior secured debt | EBITDA Multiple | Enterprise Value | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		6.3
Level 3 | Senior secured debt | EBITDA Multiple | Enterprise Value | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		8.3
Level 3 | Senior secured debt | EBITDA Multiple | Enterprise Value | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		7.3
Level 3 | Subordinated debt | Market Yield | Market Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 269,340	$ 265,289
Level 3 | Subordinated debt | Market Yield | Market Yield | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.050	0.030
Level 3 | Subordinated debt | Market Yield | Market Yield | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.140	0.120
Level 3 | Subordinated debt | Market Yield | Market Yield | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.093	0.071
Level 3 | Subordinated debt | Revenue Multiple | Enterprise Value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 65,884
Level 3 | Subordinated debt | Revenue Multiple | Enterprise Value | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.9
Level 3 | Subordinated debt | Revenue Multiple | Enterprise Value | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	8.1
Level 3 | Subordinated debt | Revenue Multiple | Enterprise Value | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	8.0
Level 3 | Subordinated debt | Transaction Price | Transaction Precedent
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 71,747
Level 3 | Common equity and warrants & preferred equity | Revenue Multiple | Enterprise Value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 10,290	$ 14,236
Level 3 | Common equity and warrants & preferred equity | Revenue Multiple | Enterprise Value | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.9	2.2
Level 3 | Common equity and warrants & preferred equity | Revenue Multiple | Enterprise Value | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	8.0	5.0
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	1.8	2.6
Level 3 | Common equity and warrants & preferred equity | EBITDA Multiple | Enterprise Value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment owned, fair value	$ 11,571	$ 17,384
Level 3 | Common equity and warrants & preferred equity | EBITDA Multiple | Enterprise Value | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.5	6.5
Level 3 | Common equity and warrants & preferred equity | EBITDA Multiple | Enterprise Value | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	14.3	14.3
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	11.0	12.8

[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 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.