v2.4.0.6
Stock-based compensation Stock-based compensation (Details 4) (Performance Units [Member], USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Performance Units [Member]
     
Weighted-average assumptions and resulting weighted-average grant date fair values [Abstract]      
Closing price of our common stock on grant date $ 68.75 $ 51.67 $ 56.90
Expected volatility 22.90% 32.80% 34.70%
Risk-free interest rate 0.50% 1.20% 1.30%
Expected dividend yield 2.20% 0.10% 0.00%
Fair value of unit (in usd per share) $ 78.21 $ 49.50 $ 62.06