Equity Award Plans - Summary of Activity for Restricted Common Stock, RSAs and RSUs and Summary of Information Related to Unvested Restricted Common Stock, RSAs and RSUs and those Expected to Vest (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Number of Shares
Beginning balance (shares)	21,697
Granted (shares)	14,654
Vested (shares)	(8,150)
Cancelled (shares)	(6,568)
Ending balance (shares)	21,633	21,697
Unvested awards subject to performance condition (shares)	1,574
Weighted- Average Grant Date Fair Value (per share)
Beginning balance (usd per share)	$ 16.07
Granted (usd per share)	15.03
Vested (usd per share)	15.20
Cancelled (usd per share)	16.54
Ending balance (usd per share)	15.34	$ 16.07
Unvested awards subject to performance condition (usd per share)	$ 17.86
Weighted- Average Contractual Life (years)
Balance	1 year 4 months 24 days	1 year 2 months 12 days
Unvested awards subject to performance condition	1 year 7 months 6 days
Aggregate Intrinsic Value
Balance	$ 267,163	$ 358,651
Unvested awards for which the requisite service period has not been rendered and vesting is subject to the achievement of a performance condition — September 30, 2020	$ 19,435