Income Taxes - Provision and Components of Net Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Provision for income taxes	$ 0	$ 0	$ 0
Domestic and foreign components of net loss
Domestic	(34,556)	(34,977)	(10,483)
Foreign	(2,401)	(2,200)	(4,375)
Total net loss	$ (36,957)	$ (37,177)	$ (14,858)