Income Taxes - Schedule of Change In Unrecognized Tax Benefit (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Unrecognized Tax Benefits, Beginning Balance	$ 488	$ 333
Reduction for prior year tax positions	(4)
Additions for prior year tax positions		6
Additions for current year tax positions	51	149
Unrecognized Tax Benefits, Ending Balance	$ 535	$ 488