Income Taxes - Summary of Reconciliation of the Beginning and Ending Amount of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Changes of Unrecognized Tax Benefits [Roll Forward]
Balance at beginning of year	$ 542	$ 455
Additions based on tax positions related to current year	186	163
Additions based on tax positions of prior year	0	0
Reductions for tax positions of prior years	(5)	0
Reductions due to expiration of statute of limitations	(73)	(76)
Balance at end of year	$ 650	$ 542