Summary of Significant Accounting Policies - Components of Other Expense, Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Abstract]
Interest expense, net	$ 123	$ 100	$ 18
Accretion of contingent earnout consideration	295	2,182	1,710
Loss on foreign exchange transactions	(116)	47	285
Other expense, net	$ 302	$ 2,327	$ 2,013