Stock-Based Compensation - Summary of Activity under Stock Option Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Number of Shares Underlying Outstanding Options
Beginning Balance (in shares)	15,140,579
Exercised (in shares)	(9,065,976)
Forfeited and canceled (in shares)	(1,216,013)
Ending Balance (in shares)	4,858,590	15,140,579
Vested and exercisable (in shares)	3,876,252
Vested and expected to vest (in shares)	4,858,590
Weighted-Average Exercise Price
Beginning Balance (in dollars per share)	$ 3.61
Exercised (in dollars per share)	3.42
Forfeited and canceled (in dollars per share)	4.08
Ending Balance (in dollars per share)	3.83	$ 3.61
Vested and exercisable (in dollars per share)	3.74
Vested and expected to vest (in dollars per share)	$ 3.83
Weighted-Average Remaining Contractual Term (Years)	5 years 9 months 18 days	6 years 2 months 8 days
Weighted-Average Remaining Contractual Term (Years), Vested and exercisable	5 years 6 months 18 days
Weighted-Average Remaining Contractual Term (Years), Vested and expected to vest	5 years 9 months 18 days
Aggregate Intrinsic Value (in thousands), Vested and exercisable	$ 119,303
Aggregate Intrinsic Value (in thousands), Vested and expected to vest	149,046
Aggregate Intrinsic Value (in thousands)	$ 149,046	$ 106,967