Underwriting, Acquisition and General Expenses (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
Underwriting Acquisition And Insurance Expenses [Abstract]
Commissions	$ 12.7	$ 141.7	$ 89.6	$ 280.2
Other underwriting and insurance expenses	33.8	268.3	260.3	401.0
Amortization of value of business acquired and other intangible assets	37.9	0.0	172.0	0.0
Underwriting, acquisition and insurance expenses, total	84.4	410.0	521.9	681.2
Net deferral of policy acquisition costs	(7.2)	9.0	(44.9)	(10.5)
Total underwriting, acquisition and general expenses	$ 77.2	$ 419.0	$ 477.0	$ 670.7