Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (23.3)	$ 42.7	$ (71.3)	$ 52.9
Income tax provision (benefit)	(4.4)	7.0	(13.0)	12.2
Net unrealized investment gains (losses), net of income taxes	(18.9)	35.7	(58.3)	40.7
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	(23.0)	42.5	(70.7)	66.9
Other investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	0.0	0.2	0.0	(14.0)
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (0.3)	$ 0.0	$ (0.6)	$ 0.0