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Stock-based compensation Stock-based compensation (Summary of Stock Options) (Details) - Share-based Payment Arrangement, Option [Member] - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Closing price of our common stock on grant date (in usd per share) $ 236.36 $ 177.31 $ 177.46
Volatility 28.10% 23.50% 24.60%
Expected life (in years) 5 years 9 months 18 days 5 years 9 months 18 days 5 years 9 months 18 days
Risk-free interest rate 0.40% 2.40% 2.80%
Expected dividend yield 3.00% 3.10% 2.90%
Granted (in usd per share) $ 42.34 $ 30.47 $ 34.60