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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Number of Shares      
Balance at the beginning of the period (in shares) 15,810,000.0 15,181,100 15,497,500
Options, Granted (in shares) 4,091,000.0 2,348,000.0 3,695,300
Options, Exercised (in shares) (1,967,000.0) (1,398,000.0) (2,878,700)
Options, Forfeited (in shares) (879,000.0) (313,100) (1,133,000.0)
Options, Expired (in shares) (331,000.0) (8,000.0)  
Balance at the end of the period (in shares) 16,724,000.0 15,810,000.0 15,181,100
Vested and unvested expected to vest as of the end of the period (in shares) 16,111,200    
Exercisable at the end of the period (in shares) 10,646,500    
Weighted Average Exercise Price      
Balance at the beginning of the period (in dollars per share) $ 8.63 $ 7.48 $ 7.37
Options, Granted (in dollars per share) 10.29 14.96 7.17
Options, Exercised (in dollars per share) 5.83 6.54 5.67
Options, Forfeited (in dollars per share) 11.07 9.55 9.55
Options, Expired (in dollars per share) 13.38 10.76  
Balance at the end of the period (in dollars per share) 9.15 $ 8.63 $ 7.48
Vested and unvested expected to vest as of the end of the period (in dollars per share) 9.08    
Exercisable at the end of the period (in dollars per share) $ 8.33    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]      
Balance at the end of the period 6 years 4 months 24 days    
Vested and unvested expected to vest at the end of the period 6 years 3 months 18 days    
Exercisable at the end of the period 5 years 1 month 6 days    
Aggregate intrinsic value of options outstanding $ 30.8    
Aggregate intrinsic value of options vested and unvested expected to vest 30.6    
Aggregate intrinsic value of options exercisable $ 27.0