Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (177.8)	$ (66.1)
Income tax provision (benefit)	(33.9)	(12.6)
Net unrealized investment gains (losses), net of income taxes	(143.9)	(53.5)
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	(177.9)	(65.7)
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 0.1	$ (0.4)