Income Taxes - Summary of Change in Gross Unrecognized Tax Positions (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Beginning balance	$ 1,048	$ 862	$ 606
Gross increases for tax positions related to current periods	143	186	256
Gross increases for tax positions related to prior periods	11	0	0
Ending balance	$ 1,202	$ 1,048	$ 862