Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 42.7	$ (4.6)	$ 52.9	$ 99.0
Income tax provision (benefit)	7.0	(0.4)	12.2	16.1
Net unrealized investment gains (losses), net of income taxes	35.7	(4.2)	40.7	82.9
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	42.5	(3.8)	66.9	95.3
Other investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	0.2	0.0	(14.0)	4.4
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 0.0	$ (0.8)	$ 0.0	$ (0.7)