Reinsurance - Components of Net Realized Capital Gains (Losses) on Funds Withheld Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Reinsurance
Net realized capital gains (losses)	$ (29.2)	$ (55.4)
Fixed maturities, available-for-sale
Reinsurance
Net realized capital gains (losses)	(30.2)	(57.0)
Fixed maturities, trading
Reinsurance
Net realized capital gains (losses)		(0.1)
Equity securities
Reinsurance
Net realized capital gains (losses)		(1.2)
Derivatives
Reinsurance
Net realized capital gains (losses)	$ 1.0	$ 2.9