Income Taxes - Aggregate Changes in Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 967	$ 822
Increases for tax positions taken during current period	152	145	$ 249
Increases for tax positions taken in prior periods			573
Decreases for tax positions taken during current period	(12)
Balance at end of year	$ 1,107	$ 967	$ 822