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Share-based payments (Summary of Stock Option Pricing Assumptions) (Details)
12 Months Ended
Sep. 11, 2018
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Stock Option Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Risk free interest rate   0.70% 2.50% 2.70%
Expected dividend yield   1.96% 2.06% 2.11%
Expected lives of the stock options   5 years 5 years 5 years
Expected volatility   28.00% 26.80% 28.10%
Performance Share Units, Equity Classified Awards [Member] | Senior Executive and Employee Performance Share Unit Plans [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Risk free interest rate       1.92%
Expected dividend yield       2.09%
Expected lives of the stock options       3 years
Expected volatility       31.10%
Average expected volatility of comparable companies       34.10%
Performance Share Units [Member] | Sign-on Grant Performance Share Unit Plans [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Risk free interest rate 2.70%      
Expected dividend yield 1.53%      
Expected lives of the stock options 1 year      
Expected volatility 29.20%