Portfolio Investments - Schedule of Financial Instruments Carried at Fair Value (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Jun. 30, 2026	Sep. 30, 2025
Schedule of Investments [Line Items]
Total investments at fair value	$ 6,796,814	$ 6,899,031	[1]
Cash equivalents	90,085	210,587
Derivative assets	18,216	24,182
Total assets at fair value	6,905,115	7,133,800
Derivative liabilities	4,577	0
Total liabilities at fair value	4,577
Investment, cost	$ 6,897,681	$ 6,863,940	[1]
Investment Owned, At Cost | Investment Type Concentration Risk
Schedule of Investments [Line Items]
% of Total Investments	100.00%	100.00%
Senior secured debt
Schedule of Investments [Line Items]
Total investments at fair value	$ 6,280,474	$ 6,438,079
Investment, cost	$ 6,382,331	$ 6,414,699
Senior secured debt | Investment Owned, At Cost | Investment Type Concentration Risk
Schedule of Investments [Line Items]
% of Total Investments	92.53%	93.45%
Subordinated debt (including CLO notes and credit linked notes)
Schedule of Investments [Line Items]
Total investments at fair value	$ 481,229	$ 429,299
Investment, cost	$ 481,298	$ 423,818
Subordinated debt (including CLO notes and credit linked notes) | Investment Owned, At Cost | Investment Type Concentration Risk
Schedule of Investments [Line Items]
% of Total Investments	6.98%	6.17%
Common equity and warrants (including equity interests in EDL JV)
Schedule of Investments [Line Items]
Total investments at fair value	$ 22,178
Common equity and warrants
Schedule of Investments [Line Items]
Total investments at fair value	9,030	$ 12,995
Investment, cost	$ 11,028	$ 11,343
Common equity and warrants | Investment Owned, At Cost | Investment Type Concentration Risk
Schedule of Investments [Line Items]
% of Total Investments	0.16%	0.17%
Preferred equity
Schedule of Investments [Line Items]
Total investments at fair value	$ 12,933	$ 18,658
Investment, cost	$ 9,920	$ 14,080
Preferred equity | Investment Owned, At Cost | Investment Type Concentration Risk
Schedule of Investments [Line Items]
% of Total Investments	0.14%	0.21%
Level 1
Schedule of Investments [Line Items]
Total investments at fair value	$ 102	$ 33
Cash equivalents	90,085	210,587
Derivative assets	0	0
Total assets at fair value	90,187	210,620
Derivative liabilities	0
Total liabilities at fair value	0
Level 1 | Senior secured debt
Schedule of Investments [Line Items]
Total investments at fair value	0	0
Level 1 | Subordinated debt (including CLO notes and credit linked notes)
Schedule of Investments [Line Items]
Total investments at fair value	0	0
Level 1 | Common equity and warrants (including equity interests in EDL JV)
Schedule of Investments [Line Items]
Total investments at fair value	102
Level 1 | Common equity and warrants
Schedule of Investments [Line Items]
Total investments at fair value		33
Level 1 | Preferred equity
Schedule of Investments [Line Items]
Total investments at fair value	0	0
Level 2
Schedule of Investments [Line Items]
Total investments at fair value	756,040	1,582,035
Cash equivalents	0	0
Derivative assets	18,216	24,182
Total assets at fair value	774,256	1,606,217
Derivative liabilities	4,577
Total liabilities at fair value	4,577
Level 2 | Senior secured debt
Schedule of Investments [Line Items]
Total investments at fair value	681,782	1,418,025
Level 2 | Subordinated debt (including CLO notes and credit linked notes)
Schedule of Investments [Line Items]
Total investments at fair value	74,258	164,010
Level 2 | Common equity and warrants (including equity interests in EDL JV)
Schedule of Investments [Line Items]
Total investments at fair value	0
Level 2 | Common equity and warrants
Schedule of Investments [Line Items]
Total investments at fair value		0
Level 2 | Preferred equity
Schedule of Investments [Line Items]
Total investments at fair value	0	0
Level 3
Schedule of Investments [Line Items]
Total investments at fair value	6,027,524	5,316,963
Cash equivalents	0	0
Derivative assets	0	0
Total assets at fair value	6,027,524	5,316,963
Derivative liabilities	0
Total liabilities at fair value	0
Level 3 | Senior secured debt
Schedule of Investments [Line Items]
Total investments at fair value	5,598,692	5,020,054
Level 3 | Subordinated debt (including CLO notes and credit linked notes)
Schedule of Investments [Line Items]
Total investments at fair value	406,971	265,289
Level 3 | Common equity and warrants (including equity interests in EDL JV)
Schedule of Investments [Line Items]
Total investments at fair value	8,928
Level 3 | Common equity and warrants
Schedule of Investments [Line Items]
Total investments at fair value		12,962
Level 3 | Preferred equity
Schedule of Investments [Line Items]
Total investments at fair value	12,933	$ 18,658
Measured at Net Asset Value
Schedule of Investments [Line Items]
Total investments at fair value	13,148
Cash equivalents	0
Derivative assets	0
Total assets at fair value	13,148
Derivative liabilities	0
Total liabilities at fair value	0
Measured at Net Asset Value | Senior secured debt
Schedule of Investments [Line Items]
Total investments at fair value	0
Measured at Net Asset Value | Subordinated debt (including CLO notes and credit linked notes)
Schedule of Investments [Line Items]
Total investments at fair value	0
Measured at Net Asset Value | Common equity and warrants (including equity interests in EDL JV)
Schedule of Investments [Line Items]
Total investments at fair value	13,148
Measured at Net Asset Value | Preferred equity
Schedule of Investments [Line Items]
Total investments at fair value	$ 0

[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.