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Share based Compensation - Stock Option Activity (Details) - Options to purchase common stock - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]      
Balance at the beginning of the period (in shares) 15,181,100 15,497,500 11,729,200
Options, Granted (in shares) 2,348,000 3,695,300 5,114,100
Options, Exercised (in shares) (1,398,000) (2,878,700) (723,100)
Options, Forfeited (in shares) (313,100) (1,133,000) (622,700)
Options, Expired (in shares) (8,000)    
Balance at the end of the period (in shares) 15,810,000 15,181,100 15,497,500
Vested and unvested expected to vest as of the end of the period (in shares) 15,152,600    
Exercisable at the end of the period (in shares) 9,977,000    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]      
Balance at the beginning of the period (in dollars per share) $ 7.48 $ 7.37 $ 7.11
Options, Granted (in dollars per share) 14.96 7.17 7.67
Options, Exercised (in dollars per share) 6.54 5.67 4.20
Options, Forfeited (in dollars per share) 9.55 9.55 8.62
Options, Expired (in dollars per share) 10.76    
Balance at the end of the period (in dollars per share) 8.63 $ 7.48 $ 7.37
Vested and unvested expected to vest as of the end of the period (in dollars per share) 8.51    
Exercisable at the end of the period (in dollars per share) $ 7.43    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]      
Balance at the end of the period 6 years 8 months    
Vested and unvested expected to vest at the end of the period 6 years 7 months    
Exercisable at the end of the period 5 years 9 months    
Aggregate intrinsic value of options outstanding $ 37.6    
Aggregate intrinsic value of options vested and unvested expected to vest 36.8    
Aggregate intrinsic value of options exercisable $ 28.8