Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 52.5	$ (19.2)	$ 20.9	$ 26.7
Income tax provision (benefit)	11.2	(3.7)	4.4	4.7
Net unrealized investment gains (losses), net of income taxes	41.3	(15.5)	16.5	22.0
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	52.0	(19.5)	21.0	26.9
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 0.5	$ 0.3	$ (0.1)	$ (0.2)