Portfolio Investments - Narrative (Details) $ in Thousands			1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2026 USD ($) borrower		May 31, 2026 director	Jun. 30, 2026 USD ($) borrower		Jun. 30, 2026 USD ($) borrower		Jun. 30, 2026 EUR (€) borrower	Sep. 30, 2025 USD ($) borrower		Sep. 30, 2025 EUR (€) borrower	Jul. 15, 2025	Jul. 23, 2024	Nov. 14, 2023
Debt Instrument [Line Items]
Total investments at fair value	$ 6,796,814			$ 6,796,814		$ 6,796,814			$ 6,899,031	[1]
Number of borrowers | borrower	152			152		152		152	158		158
Total Assets	$ 7,194,665			$ 7,194,665		$ 7,194,665			$ 7,282,434
Unfunded commitment	824,572			824,572		824,572			931,496
Investment, cost	6,897,681			6,897,681		6,897,681			6,863,940	[1]
Debt security, held-to-maturity, sold, realized gain						300
Control Investments
Debt Instrument [Line Items]
Total investments at fair value	13,148	[2],[3]		13,148	[2],[3]	13,148	[2],[3]		0
Investment, cost	13,104	[2],[3]		13,104	[2],[3]	13,104	[2],[3]		0
EDL JV LLC
Debt Instrument [Line Items]
Investment, cost	$ 13,100			$ 13,100		$ 13,100
Equity investment in EDL JV
Debt Instrument [Line Items]
Total investments at fair value | €								€ 106,560,000
Number of borrowers | borrower	15			15		15		15
Number of Directors on the Board | director			8
Number of Directors selected by the Company | director			4
Number of Directors selected by Pantheon | director			4
Number of Directors required for quorum, selected by the company | director			1
Number Directors required for a quorum, selected by Pantheon | director			1
Investment owned, ownership (as a percent)	25.00%			25.00%		25.00%		25.00%
Total Assets | €								€ 109,547,000
Equity method investments, fair value	$ 13,100			$ 13,100		$ 13,100
Investment company, financial support to investee contractually required and provided, amount | €											€ 46,000,000.0
Unfunded commitment	36,100			36,100		$ 36,100					41,700,000
Investment, cost | €								€ 106,433,000
Dividend income	$ 0
Debt securities, held-to-maturity, sale				$ 117,900
Equity investment in EDL JV | EDL JV Notes
Debt Instrument [Line Items]
Investment company, financial support to investee contractually required and provided, amount | €											11,500,000
Equity investment in EDL JV | EDL JV LLC
Debt Instrument [Line Items]
Unfunded commitment									36,100		€ 30,200,000
Equity investment in EDL JV | Pantheon
Debt Instrument [Line Items]
Investment owned, ownership (as a percent)	75.00%			75.00%		75.00%		75.00%
Line of Credit
Debt Instrument [Line Items]
Aggregate amount	$ 3,140,000			$ 3,140,000		$ 3,140,000			$ 2,735,000
Notes due 2028 | Unsecured Notes
Debt Instrument [Line Items]
Debt interest rate (as a percent)														8.40%
Notes Due 2029 | Unsecured Notes
Debt Instrument [Line Items]
Debt interest rate (as a percent)													6.50%
Notes Due 2030 | Unsecured Notes
Debt Instrument [Line Items]
Debt interest rate (as a percent)												6.19%	6.19%
JV Facility | Line of Credit | Equity investment in EDL JV | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate amount | €								€ 125,000,000.0
Long-term debt	$ 63,000			$ 63,000		$ 63,000
JV Facility | Line of Credit | Equity investment in EDL JV | Revolving Credit Facility | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread, variable rate (as a percent)						1.75%
JV Facility | Line of Credit | Equity investment in EDL JV | Revolving Credit Facility | EURIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread, variable rate (as a percent)						1.70%

[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.

[2] As defined in the Investment Company Act, the Company is deemed to be both an "Affiliated Person" of and to "Control" these portfolio companies as the Company owns more than 25% of the portfolio company's outstanding voting securities or has the power to exercise control over management or policies of such portfolio company (including through a management agreement). See Schedule 12-14 in the accompanying notes to the Consolidated Financial Statements for transactions during the nine months ended June 30, 2026 in which the issuer was both an Affiliated Person and a portfolio company that the Company is deemed to control.

[3] 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 or maintains greater than 50% of the board representation.