Segment Information - Revenues from exited business (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenues from exited business:
Change in fair value of funds withheld embedded derivative		$ 925.6	$ 1,237.7	$ 392.9	$ 4,305.0
Net realized capital gains (losses) on funds withheld assets	[1]	1.0	8.5	119.8	697.5
Reconciling Items
Revenues from exited business:
Change in fair value of funds withheld embedded derivative		925.6	1,237.7	392.9	4,305.0
Net realized capital gains (losses) on funds withheld assets		1.0	8.5	119.8	697.5
Amortization of reinsurance gain		1.6	3.4	4.5	8.3
Other impacts of reinsured business		(3.3)		(9.0)
Total revenues from exited business		$ 924.9	$ 1,249.6	$ 508.2	$ 5,010.8

[1] Includes realized and unrealized gains (losses). See Note 3, Investments, for further details.