Liability for Loss and LAE - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance	$ 907,736	$ 957,703	$ 2,497,037	$ 2,366,491
Typhoon Faxai
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance	47,800		47,800
Hurricane Dorian
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Losses incurred, net of reinsurance	$ 19,200		$ 19,200