Summary of Significant Accounting Policies - Calculation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (14,528)	$ (11,148)	$ (9,377)	$ (8,485)	$ (7,884)	$ (5,920)	$ (4,692)	$ (5,056)	$ (43,538)	$ (23,552)	$ (17,100)
Weighted-average number of common shares outstanding:
Weighted-average shares of common stock outstanding, basic and diluted									19,833	1,620	1,312
Net loss per share, basic and diluted	$ (0.35)	$ (0.34)	$ (4.17)	$ (4.19)	$ (4.08)	$ (3.60)	$ (3.18)	$ (3.56)	$ (2.20)	$ (14.54)	$ (13.03)