Reinsurance - Schedule of Reinsurance Premium (Detail) - USD ($) $ in Millions	2 Months Ended	11 Months Ended	12 Months Ended
Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2024	Dec. 31, 2022
Insurance [Abstract]
Direct written premiums	$ 186.0	$ 1,907.9	$ 1,740.6	$ 2,682.8
Reinsurance ceded to other companies	(66.8)	(803.6)	(775.5)	(1,106.7)
Reinsurance assumed from other companies	5.1	40.4	5.2	165.4
Net written premiums	124.3	1,144.7	970.3	1,741.5
Direct earned premiums	252.3	1,906.5	1,843.3	2,757.8
Reinsurance ceded to other companies	(98.4)	(724.9)	(773.6)	(1,205.3)
Reinsurance assumed from other companies	8.4	44.3	20.1	187.9
Net earned premiums	$ 162.3	$ 1,225.9	$ 1,089.8	$ 1,740.4
Percentage of reinsurance assumed to net earned premiums	5.20%	3.60%	1.80%	10.80%

Amount of earned premiums assumed from other entities.

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Amount of written premiums assumed from other entities.

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Amount of earned premiums ceded to other entities.

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Amount of written premiums ceded to other entities.

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Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

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Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

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Percentage of assumed premium earned to premium earned after effects of reinsurance, as disclosed in supplementary reinsurance information.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.

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