Loss Reserves - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Increased in incurred losses and LAE	$ 35,800
Net prior accident year reserve strengthening	3,320	$ 15,115
Increase in losses and loss adjustment expense payments	28,500
Non-catastrophe
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	11,800
Catastrophe
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	$ 8,500