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Stock-based compensation - Stock Options Outstanding (Details) - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]      
Options exercisable, Weighted-average exercise price (usd per share)   $ 25.67 $ 27.27
Options exercisable (shares)   6,714,283 9,302,140
$ 18 to 31      
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]      
Range of exercise prices, lower range (usd per share) $ 18    
Range of exercise prices, upper range (usd per share) $ 31    
Options outstanding (shares) 4,143,575    
Options outstanding, Weighted-average remaining contractual life (in years) 1 year 4 months 24 days    
Options outstanding, Weighted-average exercise price (usd per share) $ 26.03    
Options exercisable, Weighted-average exercise price (usd per share) $ 26.03    
Options exercisable (shares) 4,143,575