Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other:	$ 9.4	$ 8.0
Realized losses on fixed maturities and other:	(3.1)	(28.2)
Credit losses on fixed maturities	0.0	0.1
Other	0.0	(3.6)
Total other net losses recognized on fixed maturities and other	(0.5)	(4.2)
Net realized gains (losses) on equity securities	0.4	0.3
Change in unrealized gains (losses) on equity securities held at the end of the period	0.0	6.2
Net gains (losses) on equity securities	0.4	6.5
Net investment and other gains (losses) before income taxes	6.2	(17.9)
Income tax (benefit) provision	1.6	(5.1)
Net investment and other gains (losses), net of income taxes	4.6	(12.8)
Fixed maturities
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other:	6.8	0.2
Realized losses on fixed maturities and other:	(0.3)	(22.4)
Other investments
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other:	2.6	7.8
Realized losses on fixed maturities and other:	(2.8)	(5.8)
Other than temporary impairment losses
Schedule of Investments [Line Items]
Credit losses on fixed maturities	(0.5)	(0.6)
Impairment Related to Change in Intent
Schedule of Investments [Line Items]
Impairment related to change in intent	$ 0.0	$ 0.0