Reserves for Losses and Loss Adjustment Expenses - Impact from (Favorable) Unfavorable Development of Prior Accident Years' Loss and LAE Reserves on Each Reporting Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development	$ 48.6	$ 3.4
U.S. Operations
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development	39.7	5.0
International Operations
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development	7.8	(3.0)
Run-off Lines
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development	$ 1.1	$ 1.4