Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 43.7	$ 164.1	$ 55.3
Realized losses on fixed maturities and other	(42.8)	(121.0)	(43.0)
Credit losses on fixed maturities	0.6	(39.9)	0.0
Other	(6.3)	0.0	0.0
Net (losses) recognized on fixed maturities and other	(5.7)	(39.9)	(20.3)
Net realized gains (losses) on equity securities	71.5	(17.1)	128.9
Change in fair value recognized	(34.1)	10.3	(40.8)
Net realized gains (losses) on equity securities	37.4	(6.8)	88.1
Net realized investment (gains) losses	32.6	(3.6)	80.1
Income tax (benefit) provision	6.2	1.3	16.2
Net realized investment gains (losses), net of income taxes	26.4	(4.9)	63.9
Fixed maturities
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	30.6	37.1	22.2
Realized losses on fixed maturities and other	(11.8)	(35.2)	(11.7)
Other investments, including short-terms
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	9.8	93.8	0.1
Realized losses on fixed maturities and other	(18.5)	(78.6)	(31.3)
Other assets
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	3.3	33.2	33.0
Realized losses on fixed maturities and other	(12.5)	(7.2)	0.0
Other-than-temporary impairment losses
Schedule of Investments [Line Items]
Credit losses on fixed maturities	(0.6)	39.9	0.0
Other-than-temporary impairment losses	$ 0.0	$ 0.0	$ (20.3)