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Stock-based compensation - Summary of Stock Option Activity (Details) - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Shares subject to option      
Beginning Balance (shares) 6,714,283    
Granted (shares) 0 0 0
Exercised (shares) (2,543,235)    
Canceled/Expired (shares) (27,473)    
Ending Balance (shares) 4,143,575 6,714,283  
Vested and expected to vest (shares) 4,143,575    
Exercisable (shares) 4,143,575    
Weighted-average exercise price      
Beginning Balance (usd per share) $ 25.67    
Granted (usd per share) 0    
Exercised (usd per share) 25.14    
Canceled/Expired (usd per share) 21.93    
Ending Balance (usd per share) 26.03 $ 25.67  
Vested and expected to vest (usd per share) 26.03    
Exercisable (usd per share) $ 26.03    
Weighted-average remaining contractual term (in years)      
Weighted- average remaining contractual term (in years) 1 year 4 months 24 days 2 years  
Vested and expected to vest 1 year 4 months 24 days    
Exercisable 1 year 4 months 24 days