Organization and Summary of Significant Accounting Policies - Net Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (58,434)	$ (73,643)	$ (62,887)	$ (83,053)	$ (65,606)	$ (58,431)	$ (39,838)	$ (72,580)	$ (278,017)	$ (236,455)	$ (215,524)
Denominator:
Weighted-average common shares outstanding									62,808	56,452	55,076
Less: weighted-average common shares subject to forfeiture									(463)	(842)	(1,107)
Weighted-average common shares used to compute basic and diluted net loss per share									62,345	55,610	53,969
Basic and diluted net loss per share	$ (0.92)	$ (1.16)	$ (1.00)	$ (1.40)	$ (1.17)	$ (1.05)	$ (0.72)	$ (1.32)	$ (4.46)	$ (4.25)	$ (3.99)