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Stock-Based Compensation - Summary of Activity under Stock Option Plans (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares Underlying Outstanding Options        
Beginning Balance (in shares) 23,607,746 20,929,502 20,710,510  
Granted (in shares) 4,468,523 5,803,596 2,624,450  
Exercised (in shares) (3,522,631) (1,554,944) (272,591)  
Forfeited and canceled (in shares) (779,359) (1,570,408) (2,132,867)  
Ending Balance (in shares) 23,774,279 23,607,746 20,929,502 20,710,510
Vested and exercisable (in shares) 13,774,468      
Vested and expected to vest (in shares) 23,774,279      
Weighted-Average Exercise Price        
Beginning Balance (in dollars per share) $ 3.10 $ 2.85 $ 2.79  
Granted (in dollars per share) 5.88 3.58 3.20  
Exercised (in dollars per share) 2.32 1.64 0.99  
Forfeited and canceled (in dollars per share) 3.80 2.98 2.93  
Ending Balance (in dollars per share) 3.71 $ 3.10 $ 2.85 $ 2.79
Vested and exercisable (in dollars per share) 3.19      
Vested and expected to vest (in dollars per share) $ 3.71      
Weighted-Average Remaining Contractual Term (Years) 7 years 1 month 6 days 7 years 4 months 20 days 7 years 6 months 29 days 8 years 3 months 11 days
Weighted-Average Remaining Contractual Term (Years), Vested and exercisable 6 years 1 month 16 days      
Weighted-Average Remaining Contractual Term (Years), Vested and expected to vest 7 years 1 month 6 days      
Aggregate Intrinsic Value (in thousands), Vested and exercisable $ 205,455      
Aggregate Intrinsic Value (in thousands), Vested and expected to vest 342,262      
Aggregate Intrinsic Value (in thousands) $ 342,262 $ 22,260 $ 11,149 $ 8,634