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, 2026	Mar. 31, 2026	Dec. 31, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2026	Jun. 30, 2025
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	$ (49,539)	$ (13,610)	$ 1,342	$ (13,432)	$ 6,705	$ (17,310)	$ (61,800)	$ (24,000)
Dominion Diagnostics LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	(24,500)						(24,500)
Mosaic Companies LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	(18,600)						(18,600)
Spanx LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	(10,400)						(10,400)
All Web Leads Inc
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	(3,200)						(3,200)
Thrasio LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	7,800						7,800
Other, net
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change	$ (600)			3,000			(1,500)	5,400
Foreign currency forward contracts
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change				(15,300)			5,300	(7,300)
Telestream 2 LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change				(2,400)				(2,400)
Aurelia Netherlands Midco
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change				$ 1,300				1,300
Astra Acquisition Corp
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change							(16,800)
LABL
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change							(6,800)
Spanx LLC
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change							$ 6,900
SVP-Singer Holdings Inc.
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change								(16,600)
FinThrive Software Intermediate Holdings Inc
Schedule of Investments [Line Items]
Realized gain (loss), debt and equity securities and foreign currency transaction price change								$ (4,400)