Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	2 Months Ended	11 Months Ended	12 Months Ended		14 Months Ended
	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2024	Dec. 31, 2022	Dec. 31, 2024
Schedule of Investments [Line Items]
Total realized gains on fixed maturities and other	$ 5.5	$ 15.5	$ 18.9	$ 55.3
Total realized losses on fixed maturities and other	(5.4)	(37.0)	(8.7)	(81.0)
Impairment related to change in intent		(2.9)	0.0	(34.2)	$ 0.0
Other	0.0	(8.4)	(6.1)	(55.1)
Other net losses recognized on fixed maturities and other:	(0.2)	(13.5)	(6.9)	(93.9)
Net realized gains (losses) on equity securities	0.0	6.8	(0.9)	1.2
Change in unrealized gains (losses) on equity securities held at the end of the period	(0.2)	5.5	17.6	3.1
Net gains (losses) on equity securities	(0.2)	12.3	16.7	4.3
Net investment and other (gains) losses	(0.3)	(22.7)	20.0	(115.3)
Income tax (benefit) provision	(0.1)	(6.8)	4.2	(10.0)
Net investment and other gains (losses), net of income taxes	(0.2)	(15.9)	15.8	(105.3)
Fixed Maturities
Schedule of Investments [Line Items]
Total realized gains on fixed maturities and other	0.1	0.6	10.9	20.5
Total realized losses on fixed maturities and other	0.0	(25.7)	(1.4)	(29.9)
Other investments, including short-term investments
Schedule of Investments [Line Items]
Total realized gains on fixed maturities and other	5.4	14.9	8.0	34.8
Total realized losses on fixed maturities and other	(5.4)	(11.3)	(4.1)	(51.1)
Credit gains (losses) on fixed maturities
Schedule of Investments [Line Items]
Credit gains (losses) on fixed maturities	(0.2)	(2.2)	(0.8)	(4.6)
Other assets
Schedule of Investments [Line Items]
Total realized losses on fixed maturities and other	$ 0.0	$ 0.0	$ (3.2)	$ 0.0