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Stock-based compensation (Weighted-average Assumptions) (Details) - Performance units [Member] - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Weighted-average assumptions and resulting weighted-average grant date fair values [Abstract]      
Closing price of our common stock on grant date (in usd per share) $ 156.35 $ 164.26 $ 112.43
Volatility 25.80% 24.30% 23.80%
Risk-free interest rate 0.90% 0.80% 0.80%
Fair value of unit (in usd per share) $ 170.56 $ 182.55 $ 104.47