Net Loss Per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (23,138)	$ (29,727)
Accretion of preferred stock to redemption value		(244)
Net loss attributable to common stockholders	(23,138)	(29,971)
Less change in fair value of warrant liability	7,284	473
Net loss attributable to common stockholders for diluted loss per share	$ (30,422)	$ (30,444)
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	14,144,287	9,161,844
Assumed conversion of dilutive securities:
Private Placement Warrants (in shares)	268,408	26,743
Denominator for diluted loss per share - adjusted weighted average shares (in shares)	14,412,695	9,188,587
Net loss per share attributable to common stockholders - basic (in dollars per share)	$ (1.64)	$ (3.27)
Net loss per share attributable to common stockholders - diluted (in dollars per share)	$ (2.11)	$ (3.31)