Investments - Net Investment Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Gain (Loss) on Securities [Line Items]
Net investment losses	$ (24.0)	$ (56.5)	[1]	$ (11.0)	[1]
Fixed maturity securities
Gain (Loss) on Securities [Line Items]
Net investment losses	(20.3)	(29.1)		(7.7)
Equity securities
Gain (Loss) on Securities [Line Items]
Net investment losses	(3.9)	(32.6)		(0.8)
Short-term and other investments
Gain (Loss) on Securities [Line Items]
Net investment losses	$ 0.2	$ 5.2		$ (2.5)

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements