Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (20.3)	$ 55.4
Income tax provision (benefit)	(4.4)	9.9
Net unrealized investment gains (losses), net of income taxes	(15.9)	45.5
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	(19.9)	55.5
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (0.4)	$ (0.1)