Realized Gains or Losses and Net Unrealized Appreciation or Depreciation - Schedule of Aggregate Net Realized Gains (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	$ (17,097)	$ (136,356)	$ (33,155)
SVP-Singer Holdings Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	(16,600)
Foreign currency forward contracts
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	(11,100)	1,100	(5,800)
FinThrive Software Intermediate Holdings Inc
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	(4,500)
Telestream 2 LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	(2,400)
EOS Fitness Opco Holdings LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	11,100
Aurelia Netherlands Midco
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	1,300
Other, net
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	$ 5,100	2,900	(4,300)
Thrasio, LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		(68,500)
Pluralsight, LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		(35,300)
Impel Pharmaceuticals Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		(17,200)
All Web Leads Inc
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		(13,400)
Continental Intermodal Group LP
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		(6,800)
American Tire Distributors Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		(3,600)
P&L Development LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		(1,900)
Zephyr Bidco Limited
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		(1,700)
Lift Brands Holdings, Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		(1,400)
Alvotech
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		4,800
Ardonagh Midco 3 PLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change		$ 4,600
SIO2 Medical Products Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			(13,900)
Convergeone Holdings Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			(6,000)
Aden & Anais Merger Sub Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			(5,200)
Radiology Partners Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			(4,200)
Carvana Co.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			(2,800)
Impel Neuropharma Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			(2,300)
ASP Unifrax Holdings Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			(2,100)
WP CPP Holdings LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			(1,300)
Global Medical Response Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			(1,000)
Athenex Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			6,500
Tersera Therapeutics LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			5,200
CorEvitas Group Holdings LP
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change			$ 4,000