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Stock-based compensation plans - Valuation of Share-Based Compensation Awards (Details) - shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
SARs Liability Classified Awards      
Valuation of Share-Based Compensation Awards      
Risk-free interest rate, minimum   1.70% 1.70%
Risk-free interest rate, maximum   1.90% 1.90%
Dividend yield   0.50% 1.80%
Volatility, minimum   57.00% 14.00%
Volatility, maximum   130.00% 23.00%
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
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
Equity-settled SARs, RSUs and DSUs      
Valuation of Share-Based Compensation Awards      
Risk-free interest rate, minimum     1.90%
Risk-free interest rate, maximum     2.30%
Volatility, minimum     3.00%
Volatility, maximum     7.00%
Granted (in shares) 0 0  
Equity-settled SARs, RSUs and DSUs | Minimum      
Valuation of Share-Based Compensation Awards      
Dividend yield     2.20%
Expected lives (in years)     22 years
Equity-settled SARs, RSUs and DSUs | Maximum      
Valuation of Share-Based Compensation Awards      
Dividend yield     9.10%
Expected lives (in years)     41 years