Losses and Loss Adjustment Expenses Incurred and Paid - Information Regarding the Reserve for Unpaid Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Liability for unpaid losses and LAE at beginning of year, gross	$ 161,079	$ 122,071	$ 96,613
Reinsurance balances receivable, beginning balance	(769)	(464)	(362)
Liability for unpaid losses and LAE at beginning of year, net	160,310	121,607	96,251
Add: Provision for losses and LAE:
Current year	223,056	278,366	218,186
Prior year	(2,271)	30,636	845
Net losses and LAE incurred	220,785	309,002	219,031
Less: Losses and LAE paid:
Current year	120,708	163,792	129,216
Prior year	102,097	106,507	64,459
Net losses and LAE paid	222,805	270,299	193,675
Liability for unpaid losses and LAE at end of year, net	158,290	160,310	121,607
Reinsurance balances receivable, ending balance	840	769	464
Liability for unpaid losses and LAE at end of year, gross	$ 159,130	$ 161,079	$ 122,071