Liability for Loss and LAE - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance	$ 3,686,435	$ 3,520,431	$ 3,620,197
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
Typhoon Jebi
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		183,400
Wildfires in California
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		182,000
Hurricane Michael
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		135,200
Hurricane Florence
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		70,400
Typhoon Trami
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance		$ 33,700