Loss and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unpaid Losses And Loss Expenses [Line Items]
Loss and LAE Related to Prior Periods	$ 34,770	$ 51,306	$ 47,814
Accident Year 2013 [Member]
Unpaid Losses And Loss Expenses [Line Items]
Loss and LAE Related to Prior Periods	5,800
Accident Year 2014 [Member]
Unpaid Losses And Loss Expenses [Line Items]
Loss and LAE Related to Prior Periods	14,000
Accident Year 2015 [Member]
Unpaid Losses And Loss Expenses [Line Items]
Loss and LAE Related to Prior Periods	$ 10,100