Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 26.6	$ 46.5	$ 34.3	$ 115.5
Realized losses on fixed maturities and other	(22.9)	(32.7)	(31.2)	(97.0)
Credit gains (losses) on fixed maturity securities	0.4	(7.8)	(0.7)	(32.5)
Net realized (losses) on equity securities	0.7	(11.0)	(1.1)	(12.6)
Change in unrealized gains (losses) on equity securities held at the end of the period	20.3	15.1	35.8	(24.2)
Net realized gains (losses) on equity securities	21.0	4.1	34.7	(36.8)
Net realized investment and other gains (losses) before income taxes	24.7	17.9	37.8	(18.3)
Income tax (benefit) provision	5.2	6.7	7.9	(1.3)
Net realized investment gains (losses), net of income taxes	19.5	11.2	29.9	(17.0)
Fixed maturities
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	17.5	2.5	23.0	23.3
Realized losses on fixed maturities and other	(3.0)	(10.5)	(4.5)	(18.2)
Other investments
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	5.8	11.5	8.0	59.7
Realized losses on fixed maturities and other	(8.8)	(14.4)	(14.5)	(46.3)
Other assets
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	3.3	32.5	3.3	32.5
Realized losses on fixed maturities and other	(11.5)	0.0	(11.5)	0.0
Credit losses on fixed maturities
Schedule of Investments [Line Items]
Credit gains (losses) on fixed maturity securities	$ 0.4	$ (7.8)	$ (0.7)	$ (32.5)