Investments - Net Investment Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Gain (Loss) on Securities [Line Items]
Net investment losses	$ (17.3)	$ (24.0)	$ (56.5)	[1]
Fixed maturity securities
Gain (Loss) on Securities [Line Items]
Net investment losses	(21.1)	(20.3)	(29.1)
Equity securities
Gain (Loss) on Securities [Line Items]
Net investment losses	7.4	(3.9)	(32.6)
Short-term and other investments
Gain (Loss) on Securities [Line Items]
Net investment losses	$ (3.6)	$ 0.2	$ 5.2

[1] Recast for the adoption of ASU 2018-12.