Loss Reserves - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Increased in incurred losses and LAE	$ 16,500
Net prior accident year reserve strengthening	1,966	$ 7,085
Increase in losses and loss adjustment expense payments	22,100
Non-catastrophe
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	6,800
Catastrophe
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	$ 4,800