Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (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]
Realized gains on fixed maturities and other	$ 4.2	$ 18.0	$ 38.6	$ 133.5
Realized losses on fixed maturities and other	(5.8)	(31.0)	(37.1)	(128.0)
Credit losses on fixed maturity securities	0.8	10.5	1.5	43.0
Net realized gains (losses) on equity securities	1.4	(3.2)	0.3	(15.8)
Change in unrealized gains (losses) on equity securities held at the end of the period	(5.1)	10.5	30.7	(13.7)
Net realized gains (losses) on equity securities	(3.7)	7.3	31.0	(29.5)
Net realized investment and other gains (losses) before income taxes	(5.3)	(5.7)	32.5	(24.0)
Income tax (benefit) provision	0.0	(0.5)	0.0	(1.7)
Net realized investment gains (losses), net of income taxes	(5.3)	(5.2)	32.5	(22.3)
Fixed maturities
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	3.3	7.2	26.3	30.5
Realized losses on fixed maturities and other	(0.4)	(7.5)	(5.0)	(25.7)
Other investments
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0.9	10.8	8.9	70.7
Realized losses on fixed maturities and other	(3.3)	(12.6)	(16.6)	(58.4)
Other assets
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0.0	0.0	3.4	32.3
Realized losses on fixed maturities and other	(1.4)	(0.4)	(14.0)	(0.9)
Credit losses on fixed maturities
Schedule of Investments [Line Items]
Credit losses on fixed maturity securities	$ (0.7)	$ (10.5)	$ (1.5)	$ (43.0)