Reserves for Losses and Loss Adjustment Expenses -Schedule of Impact from (Favorable) Unfavorable Development of Prior Accident Years' Loss and LAE Reserves on Each Reporting Segment (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
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development	$ 0.0	$ 0.0	$ 267.9	$ 254.9	$ 64.7
Casualty Lines
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development		0.0		87.7
Specialty Lines
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development		0.0		3.1
U.S. Operations
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development			246.3		64.5
International Operations
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development			21.4		(2.7)
Run-off Lines
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development		$ 0.0	$ 0.2	$ 164.1	$ 2.9