Investment Portfolio - Narrative (Details) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 18, 2021 USD ($)	Sep. 30, 2024 USD ($) company	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) company	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($) company
Schedule of Investments [Line Items]
Amount funded to joint venture				$ 1,799.8
Net realized gain (loss) on investments:				(350.0)	$ (338.0)
Receivable for investments sold and repaid		$ 468.0		468.0		$ 246.0
Unfunded Debt Securities
Schedule of Investments [Line Items]
Unfunded commitments		1,600.5		1,600.5		995.1
Unfunded Equity/Other commitments
Schedule of Investments [Line Items]
Unfunded commitments		533.8		533.8		616.4
Partnership Interest
Schedule of Investments [Line Items]
Unfunded commitments		735.2		735.2		$ 560.2
FS/KKR Advisor, LLC
Schedule of Investments [Line Items]
Cost basis of investments sold				554.1
Proceeds from sale of investments				559.6
Net realized gain (loss) on investments:				5.5
Receivable for investments sold and repaid		$ 437.4		437.4
Credit Opportunities Partners JV, LLC
Schedule of Investments [Line Items]
Aggregate credit opportunities partners	$ 2,640.0
Credit opportunities partners JV, LLC, entitled to a fee percentage	0.25%
FS KKR Capital Corp.
Schedule of Investments [Line Items]
Contributed capital to committed capital ratio	87.50%
Joint venture voting control, percentage	50.00%
Amount funded to joint venture				$ 1,574.8
SCRS
Schedule of Investments [Line Items]
Contributed capital to committed capital ratio	12.50%
Joint venture voting control, percentage	50.00%
Controlled/affiliated
Schedule of Investments [Line Items]
Number of investments in portfolio companies | company		30		30		27
Net realized gain (loss) on investments:		$ 7.0	$ (2.0)	$ (1.7)	(174.0)	$ (174.8)
Non-controlled/affiliated
Schedule of Investments [Line Items]
Number of investments in portfolio companies | company		10		10		13
Net realized gain (loss) on investments:		$ (26.0)	$ (14.0)	$ (34.4)	$ (17.0)	$ (21.2)