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Stock-based compensation (Weighted-average Assumptions) (Details) - Performance units [Member] - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
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) $ 164.26 $ 112.43 $ 92.03
Volatility 24.30% 23.80% 21.00%
Risk-free interest rate 0.80% 0.80% 0.40%
Fair value of unit (in usd per share) $ 182.55 $ 104.47 $ 102.73