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Share-based payment plans - Valuation of Share-Based Compensation Awards (Details) - shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
SARs Liability Classified Awards      
Valuation of Share-Based Compensation Awards      
Risk-free interest rate, minimum 1.70% 1.70% 1.70%
Risk-free interest rate, maximum 1.90% 1.90% 1.90%
Dividend yield 0.50% 1.80% 1.80%
Granted (in shares) 0 0  
SARs Liability Classified Awards | Minimum      
Valuation of Share-Based Compensation Awards      
Expected lives (in years) 2 months 12 days 3 months 18 days 1 year
Volatility, minimum 57.00% 14.00% 14.00%
SARs Liability Classified Awards | Maximum      
Valuation of Share-Based Compensation Awards      
Expected lives (in years) 4 years 7 months 6 days 5 years 4 months 24 days 6 years 6 months
Volatility, maximum 130.00% 23.00% 25.00%
Equity-settled SARs, RSUs and DSUs      
Valuation of Share-Based Compensation Awards      
Risk-free interest rate, minimum   1.90% 0.60%
Risk-free interest rate, maximum   2.30% 1.90%
Volatility, minimum   22.00% 17.00%
Volatility, maximum   41.00% 25.00%
Granted (in shares) 0    
Equity-settled SARs, RSUs and DSUs | Minimum      
Valuation of Share-Based Compensation Awards      
Dividend yield   2.20% 1.50%
Expected lives (in years)   3 years 4 months 24 days
Equity-settled SARs, RSUs and DSUs | Maximum      
Valuation of Share-Based Compensation Awards      
Dividend yield   9.10% 2.20%
Expected lives (in years)   7 years 7 years