Realized Gains or Losses and Net Unrealized Appreciation or Depreciation - Schedule of Aggregate Net Realized Gains (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2025	Jun. 30, 2024
Schedule of Investments [Line Items]
Net realized gains (losses)	$ (13,432)	$ 6,705	$ (17,310)	$ (69,452)	$ (6,603)	$ (8,453)	$ (24,000)	$ (84,500)
Foreign currency forward contracts
Schedule of Investments [Line Items]
Net realized gains (losses)	(15,300)						(7,300)
Telestream 2 LLC
Schedule of Investments [Line Items]
Net realized gains (losses)	(2,400)						(2,400)
Aurelia Netherlands Midco
Schedule of Investments [Line Items]
Net realized gains (losses)	1,300						1,300
Other, net
Schedule of Investments [Line Items]
Net realized gains (losses)	$ 3,000			(1,000)			5,400	(1,000)
Thrasio, LLC
Schedule of Investments [Line Items]
Net realized gains (losses)				$ (68,500)				(68,500)
SVP-Singer Holdings Inc.
Schedule of Investments [Line Items]
Net realized gains (losses)							(16,600)
FinThrive Software Intermediate Holdings Inc
Schedule of Investments [Line Items]
Net realized gains (losses)							$ (4,400)
All Web Leads Inc
Schedule of Investments [Line Items]
Net realized gains (losses)								(13,400)
Continental Intermodal Group LP
Schedule of Investments [Line Items]
Net realized gains (losses)								(6,800)
P&L Development LLC
Schedule of Investments [Line Items]
Net realized gains (losses)								(1,900)
American Tire Distributors Inc.
Schedule of Investments [Line Items]
Net realized gains (losses)								(1,800)
Zephyr Bidco Limited
Schedule of Investments [Line Items]
Net realized gains (losses)								(1,700)
Lift Brands Holdings, Inc.
Schedule of Investments [Line Items]
Net realized gains (losses)								(1,400)
Alvotech
Schedule of Investments [Line Items]
Net realized gains (losses)								4,800
Ardonagh Midco 3 PLC
Schedule of Investments [Line Items]
Net realized gains (losses)								4,600
Foreign currency forward contracts
Schedule of Investments [Line Items]
Net realized gains (losses)								$ 2,600