Income Taxes - Schedule of Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate (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
Income Tax Disclosure [Abstract]
Income tax provision (benefit) at expected rate	$ 2.9	$ (24.7)	$ (40.2)	$ (14.7)
Tax effect of:
Nontaxable investment income	0.0	(0.1)	(0.2)	(0.4)
Foreign exchange adjustments	0.0	(2.6)	0.0	(2.1)
Impairment of goodwill	0.0	0.0	0.0	5.4
Base erosion and anti-abuse tax	0.0	23.5	0.0	0.0
Withholding taxes	0.0	0.1	0.0	2.7
Ireland capital loss	0.0	0.0	3.3	0.0
U.S. state tax expense, net of federal income tax effect	0.1	(1.2)	(0.2)	0.0
Change in uncertain tax position liability	(3.4)	1.2	0.0	(1.4)
Change in valuation allowance	1.8	(1.4)	(2.8)	(7.6)
Impact of change in tax rate related to Finance Act 2021	(0.2)	(0.2)	0.0	1.7
Brazil premiums and underwriting	0.0	0.0	0.0	0.3
Sale of Brazil and Malta operations	0.0	0.0	0.0	6.6
Excess executive compensation	0.0	2.0	0.9	0.0
United Kingdom debt forgiveness	0.0	0.0	0.0	1.1
Prior period adjustment	0.0	0.0	(4.5)	0.0
Other	0.1	3.7	1.5	0.4
Income tax provision (benefit)	$ 1.3	$ 0.3	$ (42.2)	$ (8.0)