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, 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	$ (65,606)	$ (58,431)	$ (39,838)	$ (72,580)	$ (50,186)	$ (59,433)	$ (40,818)	$ (65,087)	$ (236,455)	$ (215,524)	$ (285,405)
Denominator:
Weighted-average common shares outstanding									56,452	55,076	53,703
Less: weighted-average common shares subject to forfeiture									(842)	(1,107)	(1,351)
Weighted-average common shares used to compute basic and diluted net loss per share									55,610	53,969	52,352
Basic and diluted net loss per share	$ (1.17)	$ (1.05)	$ (0.72)	$ (1.32)	$ (0.92)	$ (1.10)	$ (0.76)	$ (1.22)	$ (4.25)	$ (3.99)	$ (5.45)