Income Taxes - Components of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current provision (benefit):
Federal	$ 0	$ 0	$ 0
State	5	5	5
Foreign	(13)	64	(14)
Total current provision (benefit)	(8)	69	(9)
Deferred provision (benefit):
Federal	0	0	0
State	0	0	0
Foreign	(29)	12	(31)
Total deferred provision (benefit)	(29)	12	(31)
Provision for (benefit from) income taxes	$ (37)	$ 81	$ (40)