Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other:	$ 55.3	$ 43.7	$ 164.1
Realized losses on fixed maturities and other:	(81.0)	(42.8)	(121.0)
Credit gains (losses) on fixed maturities	4.6	(0.6)	39.9
Other	(55.1)	(6.3)	0.0
Other net losses recognized on fixed maturities and other:	(93.9)	(5.7)	(39.9)
Net realized gains (losses) on equity securities	1.2	71.5	(17.1)
Change in fair value recognized	3.1	(34.1)	10.3
Net gains (losses) on equity securities	4.3	37.4	(6.8)
Net investment and other (gains) losses	(115.3)	32.6	(3.6)
Income tax (benefit) provision	(10.0)	6.2	1.3
Net investment and other gains (losses), net of income taxes	(105.3)	26.4	(4.9)
Fixed maturities
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other:	20.5	30.6	37.1
Realized losses on fixed maturities and other:	(29.9)	(11.8)	(35.2)
Other investments, including short-terms
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other:	34.8	9.8	93.8
Realized losses on fixed maturities and other:	(51.1)	(18.5)	(78.6)
Other assets
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other:	0.0	3.3	33.2
Realized losses on fixed maturities and other:	0.0	(12.5)	(7.2)
Impairment related to change in intent
Schedule of Investments [Line Items]
Impairment related to change in intent	$ (34.2)	$ 0.0	$ 0.0