Income Taxes - Provision (Benefit) for Income Taxes Differs from Amounts Computed by Applying Statutory Federal Corporate Tax Rate (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Provision (benefit) for income taxes at statutory rate		$ 2,305	$ (15,796)	$ (900)
Tax effect of:
Tax-exempt investment income		(66)	(48)	(22)
Change in the beginning of the period balance of the valuation allowance for deferred tax assets allocated to federal income taxes		1	(157)	9
Stock-based compensation		229	64	22
State income taxes, net of federal income tax benefit and state valuation allowance		243	60	142
Effect of federal tax law change	$ 12,500	12,509
Other items		(31)	29	107
Income tax expense (benefit)		$ 15,190	$ (15,848)	$ (642)