Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 12.7	$ 11.5
Realized losses on fixed maturities and other	(23.4)	(11.0)
Credit losses on fixed maturities	1.0	1.1
Other(1)	(28.5)	0.0
Net (losses) recognized on fixed maturities and other	(29.5)	(1.1)
Net realized gains (losses) on equity securities	(1.0)	(1.8)
Change in unrealized gains (losses) on equity securities held at the end of the period	6.7	15.5
Net realized gains (losses) on equity securities	5.7	13.7
Net realized investment and other gains (losses) before income taxes	(34.5)	13.1
Income tax (benefit) provision	(0.7)	2.6
Net realized investment and other gains (losses), net of income taxes	(33.8)	10.5
Fixed maturities
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	11.2	5.5
Realized losses on fixed maturities and other	(16.6)	(1.5)
Other investments
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	1.5	6.0
Realized losses on fixed maturities and other	(6.8)	(9.5)
Other than temporary impairment losses
Schedule of Investments [Line Items]
Credit losses on fixed maturities	$ (1.0)	$ (1.1)