Net Loss per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss attributable to common stockholders, basic									$ (29,076)	$ (14,068)	$ (4,608)
Net loss attributable to common stockholders, diluted									$ (29,076)	$ (14,068)	$ (4,608)
Denominator:
Weighted‑average common shares outstanding (shares)									5,285,542	5,280,058	5,060,918
Less: weighted‑average common shares subject to repurchase (shares)									(177,421)	(292,925)	(325,770)
Weighted‑average number of common shares used to compute basic and diluted net loss per share (in shares)									5,108,121	4,987,133	4,735,148
Net loss per share attributable to common stockholders:
Basic and diluted (in USD per share)	$ (1.80)	$ (1.59)	$ (1.21)	$ (1.05)	$ (0.96)	$ (0.68)	$ (0.72)	$ (0.43)	$ (5.69)	$ (2.82)	$ (0.97)