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	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ (4.1)	$ 4.4
U.S. Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	(14.8)	(28.7)
International Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	(0.8)	17.0
Run Off Lines [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ 11.5	$ 16.1