Net Loss Per Share - Weighted Average Shares (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									$ (163,458)	$ (89,195)	$ (43,847)
Accretion of Series A redeemable convertible preferred stock to redemption value											(656)
Net loss attributable to common stockholders	$ (44,472)	$ (47,015)	$ (39,390)	$ (32,581)	$ (27,183)	$ (26,362)	$ (21,239)	$ (14,411)	$ (163,458)	$ (89,195)	$ (44,503)
Denominator:
Weighted average common shares outstanding, basic and diluted									78,688,878	39,285,468	8,023,785
Net loss per share, basic and diluted	$ (0.56)	$ (0.59)	$ (0.50)	$ (0.42)	$ (0.35)	$ (0.42)	$ (2.43)	$ (1.72)	$ (2.08)	$ (2.27)	$ (5.55)