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, 2020	Mar. 31, 2019
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Net losses and adjustment expenses related to pandemic	$ 26.2
Total unfavorable (favorable) prior-year development	2.7	$ (2.5)
U.S. Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	3.3	(4.0)
International Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	(0.4)	0.8
Run Off Lines [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ (0.2)	$ 0.7