Liability for Loss and LAE - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance	$ 4,834,935	$ 4,339,046	$ 3,686,435
COVID-19 Global Pandemic
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		415,200
Hurricane Laura
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		114,200
Hurricane Sally
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		92,600
Earthquakes | Puerto Rico
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		$ 19,100
Hurricane Ida
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance	268,700
Winter Storm Uri and Other Storms
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance	252,800
Severe Flooding in Northwestern and Central Europe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance	125,200
Midwest Tornadoes
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance	$ 18,500