Net Loss Per Share - Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Earnings Per Share [Abstract]
Net loss	$ (34,414)	$ (30,428)	$ (28,784)	$ (20,954)	$ (28,627)	$ (13,334)	$ (93,627)	$ (62,914)	$ (86,500)
Net loss attributable to common stockholders	$ (34,414)			$ (20,954)			$ (93,627)	$ (62,914)
Weighted-average shares of common stock, basic and diluted	54,973,788			12,665,834			27,324,297	12,427,367
Net loss per share attributable to common stockholders, basic and diluted	$ (0.63)			$ (1.65)			$ (3.43)	$ (5.06)