Net Loss Per Share - Reconciliation of Numerator and Denominators in Computing Net Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 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
Numerator:
Net loss	$ (60,089)	$ (53,324)	$ (25,800)	$ (290,478)	$ (37,763)	$ (35,202)	$ (139,213)	$ (363,443)
Denominator:
Shares used in per share calculation (in shares)	55,548			43,029			53,039	42,577
Net loss per share, basic and diluted (in dollars per share)	$ (1.08)			$ (6.75)			$ (2.62)	$ (8.54)