Liability for Loss and LAE - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance	$ 4,339,046	$ 3,686,435	$ 3,520,431
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
Typhoon Hagibis
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		183,700
Typhoon Faxai
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		95,400
Civil Unrest in Chile
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		22,300
Hurricane Dorian
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		$ 13,600