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	6 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development		$ 61.6	$ (4.1)
U.S. Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development		(9.8)	(14.8)
International Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ 68.7	69.5	(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		$ 1.9	$ 11.5