Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (90.1)	$ (23.3)	$ (417.9)	$ (71.3)
Income tax provision (benefit)	(14.2)	(4.4)	(77.3)	(13.0)
Net unrealized investment gains (losses), net of income taxes	(75.9)	(18.9)	(340.6)	(58.3)
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	(88.9)	(23.0)	(417.6)	(70.7)
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (1.2)	$ (0.3)	$ (0.3)	$ (0.6)