Net Loss per Share - Reconciliation of the Numerator and Denominator used in Computing Basic and Diluted Net Income (Loss) Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income (loss)	$ (17,322)	$ (1,673)	$ 900
Less: Accretion of preferred stock		(6,258)	(11,301)
Net income (loss) attributable to common stockholders	$ (17,322)	$ (7,931)	$ (10,401)
Denominator:
Weighted-average common shares outstanding	20,763	10,904	4,067
Weighted-average unvested shares of common stock subject to repurchase	(83)	(126)	(174)
Weighted-average shares used to compute net income (loss) per share attributable to common stockholders — basic and diluted	20,680	10,778	3,893
Net income (loss) per share attributable to common stockholders — basic and diluted	$ (0.84)	$ (0.74)	$ (2.67)