Reserves for Losses and Loss Adjustment Expenses - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Loss adjustment expenses related to pandemic	$ 12.4
Number of reportable segments | segment	2
Claims remain outstanding period	10 years
Number of reporting segments | segment	2
Prior years	$ 138.3	$ 7.7	$ 138.1
U.S. Operations
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Prior years	120.9	2.4	15.7
U.S. Operations | Liability
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Prior years	112.1
U.S. Operations | Professional
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Prior years	33.0
International Operations
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Prior years	$ (26.9)	(6.2)	110.4
Run-off Lines
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Percentage of gross reserves for losses	5.00%
Prior years	$ 44.3	$ 11.5	$ 12.0