Income Taxes - Components of Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current tax provision:
Federal	$ 0	$ 0	$ 0
State	0	0	0
Foreign	183	170	59
Total current tax provision	183	170	59
Deferred tax provision (benefit):
Foreign	(27)	(27)	(4)
Total deferred tax provision (benefit)	(27)	(27)	(4)
Total provision for income taxes	$ 156	$ 143	$ 55