Loss Reserves - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Increased in incurred losses and LAE	$ 500
Provision for Losses and LAE for Claims Occurring in the Current Year	603,564	$ 578,272
Net prior accident year reserve strengthening	19,814	44,609
Increase in losses and loss adjustment expense payments	51,900
Catastrophe
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	$ 12,600	$ 77,900