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Share-based compensation - Schedule of management and director options outstanding (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Options outstanding (in shares) 8,306,658 8,306,658 8,888,053 8,888,053
Options outstanding, weighted average exercise price (in dollars per share) $ 13.45 $ 13.45 $ 13.43 $ 13.43
Management and director options        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Options outstanding (in shares) 8,306,658      
Options outstanding, weighted average exercise price (in dollars per share) $ 13.45      
Remaining contractual life 1 year 2 months 12 days 2 years 2 months 12 days    
Options exercisable (in shares) 8,306,658      
Options exercisable, weighted average exercise price (in dollars per share) $ 13.45      
Management and director options | $10.00 - $10.89        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Exercise price, lower limit (in dollars per share) 10.00      
Exercise price, upper limit (in dollars per share) $ 10.89      
Options outstanding (in shares) 4,774,694      
Options outstanding, weighted average exercise price (in dollars per share) $ 10.04      
Remaining contractual life 1 year 1 month 6 days      
Options exercisable (in shares) 4,774,694      
Options exercisable, weighted average exercise price (in dollars per share) $ 10.04      
Management and director options | $15.05 - $16.89        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Exercise price, lower limit (in dollars per share) 15.05      
Exercise price, upper limit (in dollars per share) $ 16.89      
Options outstanding (in shares) 1,800,866      
Options outstanding, weighted average exercise price (in dollars per share) $ 15.92      
Remaining contractual life 1 year 3 months 18 days      
Options exercisable (in shares) 1,800,866      
Options exercisable, weighted average exercise price (in dollars per share) $ 15.92      
Management and director options | $20.00 - $25.05        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Exercise price, lower limit (in dollars per share) 20.00      
Exercise price, upper limit (in dollars per share) $ 25.05      
Options outstanding (in shares) 1,731,098      
Options outstanding, weighted average exercise price (in dollars per share) $ 20.28      
Remaining contractual life 1 year 2 months 12 days      
Options exercisable (in shares) 1,731,098      
Options exercisable, weighted average exercise price (in dollars per share) $ 20.28