Net Loss per Share - Computation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (17,501)	$ (16,409)	$ (29,174)	$ (14,103)	$ (13,865)	$ (8,735)	$ (8,663)	$ (7,661)	$ (77,187)	$ (38,924)	$ (36,957)
Denominator:
Weighted-average shares used to compute net loss per common share, basic and diluted									25,894,024	22,364,515	17,694,505
Net loss per shares, basic and diluted	$ (0.63)	$ (0.61)	$ (1.18)	$ (0.58)	$ (0.57)	$ (0.38)	$ (0.41)	$ (0.36)	$ (2.98)	$ (1.74)	$ (2.09)