Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	2 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 64.6	$ 43.2	$ (384.5)	$ (106.9)
Income tax provision (benefit)	13.5	7.4	(71.7)	(21.5)
Net unrealized investment gains (losses), net of income taxes	51.1	35.8	(312.8)	(85.4)
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	64.2	41.2	(383.7)	(105.9)
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 0.4	$ 2.0	$ (0.8)	$ (1.0)