Net Loss Per Share - Basic Net Loss Per Share (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
Basic and diluted net loss per share attributable to common stockholders:
Net loss attributable to common stockholders	$ (26,017)	$ (18,778)	$ (24,453)	$ (17,124)	$ (17,399)	$ (15,010)	$ (13,804)	$ (13,765)	$ (86,372)	$ (59,978)	$ (63,386)
Weighted average common shares outstanding, basic and diluted									45,273,231	38,115,142	28,818,196
Net loss per share attributable to common stockholders, basic and diluted									$ (1.91)	$ (1.57)	$ (2.20)