Net Loss per Share - Reconciliation of the Numerator and Denominator used in Computing Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (35,231)	$ (17,322)	$ (1,673)
Less: Accretion of preferred stock			(6,258)
Net loss attributable to common stockholders	$ (35,231)	$ (17,322)	$ (7,931)
Denominator:
Weighted-average shares used to compute net loss per share attributable to common stockholders — basic and diluted	21,334	20,680	10,778
Net loss per share attributable to common stockholders — basic and diluted	$ (1.65)	$ (0.84)	$ (0.74)