Net Loss Per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net loss	$ (10,622)	$ (9,414)
Accretion of preferred stock to redemption value		(244)
Net loss attributable to common stockholders	$ (10,622)	$ (9,658)
Denominator:
Weighted average number of common shares outstanding - basic and diluted (in shares)	12,449,421	6,945,623
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (0.85)	$ (1.39)