Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other:	$ 15.1	$ 26.6	$ 27.9	$ 34.3
Realized losses on fixed maturities and other:	(26.1)	(11.8)	(49.5)	(19.0)
Credit gains (losses) on fixed maturities	0.5	(0.4)	1.6	0.7
Other(1)	(25.2)	(11.5)	(53.7)	(11.5)
Net (losses) recognized on fixed maturities and other	(27.5)	(11.1)	(57.1)	(12.2)
Net realized gains (losses) on equity securities	1.1	0.7	0.1	(1.1)
Change in unrealized gains (losses) on equity securities held at the end of the period	(3.0)	20.3	3.7	35.8
Net gains (losses) on equity securities	(1.9)	21.0	3.8	34.7
Net investment and other gains (losses) before income taxes	(40.4)	24.7	(74.9)	37.8
Income tax (benefit) provision	(1.1)	5.2	(1.8)	7.9
Net investment and other gains (losses), net of income taxes	(39.3)	19.5	(73.1)	29.9
Fixed maturities
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other:	4.7	17.5	15.9	23.0
Realized losses on fixed maturities and other:	(4.8)	(3.0)	(21.4)	(4.5)
Other investments
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other:	10.4	9.1	12.0	11.3
Realized losses on fixed maturities and other:	(21.3)	(8.8)	(28.1)	(14.5)
Other than temporary impairment losses
Schedule of Investments [Line Items]
Credit gains (losses) on fixed maturities	$ (2.3)	$ 0.4	$ (3.4)	$ (0.7)