Income Taxes - Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Beginning balance	$ 311	$ 232	$ 162
Increases based on tax positions related to prior years	0	0	0
Decreases based on tax positions related to prior years	(79)		(77)
Decreases due to ownership change	(232)
Increases based on tax positions in current year	43	79	147
Settlement	0	0	0
Lapse of statute of limitations	0	0	0
Ending balance	$ 43	$ 311	$ 232