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Stock-based compensation Stock-based compensation (Summary of Stock Options) (Details) - Employee Stock Option [Member] - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Closing price of our common stock on grant date (in usd per share) $ 162.60 $ 156.35
Volatility 22.70% 24.30%
Expected life 5 years 9 months 18 days 5 years 9 months 18 days
Risk-free interest rate 2.10% 1.50%
Expected dividend yield 2.80% 2.60%
Granted (in usd per share) $ 27.54 $ 27.55