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Stock award plan - Stock Option Activity (Details) - Stock option - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Number of options      
Outstanding at the beginning of the period (in shares) 8,534,358 6,448,642 5,854,316
Granted (in shares) 3,977,995 3,181,623 1,913,873
Exercised (in shares) (949,887) (633,973) (202,085)
Forfeited (in shares) (518,527) (461,934) (1,117,462)
Outstanding at the end of the period (in shares) 11,043,939 8,534,358 6,448,642
Vested or expected to vest at the end of the period (in shares) 5,370,703    
Exercisable at the end of the period (in shares) 5,242,221    
Weighted- average exercise price      
Outstanding at the beginning of the period (in dollars per share) $ 28.58 $ 29.00 $ 34.71
Granted (in dollars per share) 35.81 26.64 12.34
Exercised (in dollars per share) 19.25 18.61 10.80
Forfeited (in dollars per share) 35.27 35.36 33.65
Outstanding at the end of the period (in dollars per share) 31.67 $ 28.58 $ 29.00
Vested or expected to vest at the end of the period (in dollars per share) 31.62    
Exercisable at the end of the period (in dollars per share) $ 31.37    
Weighted- average remaining contractual term      
Outstanding at the end of the period 7 years 4 months 2 days  
Vested or expected to vest at the end of the period 8 years 9 months 29 days    
Exercisable at the end of the period 5 years 8 months 12 days    
Aggregate intrinsic value      
Outstanding at the end of the period (in dollars) $ 192,350  
Vested or expected to vest at the end of the period (in dollars) 88,165    
Exercisable at the end of the period (in dollars) $ 98,781