Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (106.9)	$ 82.4	$ 97.9
Income tax provision (benefit)	(21.5)	16.5	15.4
Net unrealized investment gains (losses), net of income taxes	(85.4)	65.9	82.5
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	(105.9)	96.0	93.3
Other assets
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	0.0	(14.3)	4.4
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (1.0)	$ 0.7	$ 0.2