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Stock-based compensation (Details 4) (Performance Units [Member], USD $)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Performance Units [Member]
     
Weighted-average assumptions and resulting weighted-average grant date fair values [Abstract]      
Closing price of our common stock on grant date $ 92.03 $ 68.75 $ 51.67
Expected volatility 21.00% 22.90% 32.80%
Risk-free interest rate 0.40% 0.50% 1.20%
Fair value of unit (in usd per share) $ 102.73 $ 78.21 $ 49.50