Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (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]
Realized gains on fixed maturities and other	$ 18.0	$ 11.2	$ 133.5	$ 34.9
Realized losses on fixed maturities and other	(31.0)	(15.0)	(127.4)	(42.3)
Credit losses on fixed maturity securities	(10.5)	0.0	(43.0)	0.0
Net realized (losses) gains on equity securities	(3.2)	6.4	(15.8)	7.7
Change in unrealized gains (losses) on equity securities held at the end of the period	10.5	(8.8)	(12.0)	58.0
Net realized gains (losses) on equity securities	7.3	(2.4)	(27.8)	65.7
Net realized investment and other (losses) gains before income taxes	(5.7)	(6.2)	(21.7)	58.3
Income tax (benefit) provision	(0.5)	(0.3)	(1.7)	11.9
Net realized investment (losses) gains, net of income taxes	(5.2)	(5.9)	(20.0)	46.4
Gain (Loss) on Investments
Schedule of Investments [Line Items]
Credit losses on fixed maturity securities	(10.5)		(43.0)
Fixed maturities
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	7.2	3.7	30.5	13.0
Realized losses on fixed maturities and other	(7.5)	(1.6)	(25.7)	(8.7)
Other investments
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	10.8	7.5	70.7	21.9
Realized losses on fixed maturities and other	(12.6)	(8.4)	(58.4)	(21.8)
Other assets
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0.0	0.0	32.3	0.0
Realized losses on fixed maturities and other	(0.4)	0.0	(0.3)	0.0
Credit losses on fixed maturities
Schedule of Investments [Line Items]
Credit losses on fixed maturity securities	$ (10.5)	$ (5.0)	$ (43.0)	$ (11.8)