Accumulated Other Comprehensive Income (Loss) - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2024	Dec. 31, 2022
Unrealized gains and losses on securities:
Net investment and other (gains) losses		$ 0.3	$ 22.7	$ (20.0)	$ 115.3
Income tax provision (benefit)		1.3	0.3	(42.2)	(8.0)
Total, net of taxes	$ (0.9)	(0.9)	210.4	148.1	175.2
Reclassification out of accumulated other comprehensive income
Unrealized gains and losses on securities:
Total, net of taxes		(0.1)	22.1	(6.9)	66.2
Unrealized Holding Gains (Losses) on Securities | Reclassification out of accumulated other comprehensive income
Unrealized gains and losses on securities:
Net investment and other (gains) losses		(0.1)	28.0	(8.7)	43.6
Income tax provision (benefit)		0.0	(5.9)	1.8	(9.2)
Foreign Currency Translation Adjustments | Reclassification out of accumulated other comprehensive income
Unrealized gains and losses on securities:
Net investment and other (gains) losses		$ 0.0	$ 0.0	$ 0.0	$ 31.8