Net Loss per Share Attributable to Common Stock - Computation of Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (80,805)	$ (41,978)	$ (15,002)
Accretion of redeemable convertible preferred stock	0	0	(175,915)
Net loss attributable to common stock—basic	(80,805)	(41,978)	(190,917)
Net loss attributable to common stock—diluted	$ (80,805)	$ (41,978)	$ (190,917)
Denominator:
Weighted average shares —basic and diluted (in shares)	91,583,533	85,669,039	42,722,114
Net loss per share attributable to common stock—basic and diluted (in dollars per share)	$ (0.88)	$ (0.49)	$ (4.47)