Income Taxes - Components of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current provision (benefit):
Federal	$ 0	$ 0	$ 0
State	5	5	5
Foreign	64	(14)	(13)
Total current provision (benefit)	69	(9)	(8)
Deferred provision (benefit):
Federal	0	0	(293)
State	0	0	(21)
Foreign	12	(31)	(16)
Total deferred provision (benefit)	12	(31)	(330)
Provision for (benefit from) income taxes	$ 81	$ (40)	$ (338)