Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (150.4)	$ 18.1	$ (327.8)	$ (48.0)
Income tax provision (benefit)	(29.3)	4.0	(63.1)	(8.6)
Net unrealized investment gains (losses), net of income taxes	(121.1)	14.1	(264.7)	(39.4)
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	(151.2)	18.0	(328.7)	(47.7)
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 0.8	$ 0.1	$ 0.9	$ (0.3)