Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 55.4	$ (177.8)
Income tax provision (benefit)	9.9	(33.9)
Net unrealized investment gains (losses), net of income taxes	45.5	(143.9)
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	55.5	(177.9)
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (0.1)	$ 0.1