Stock-based compensation Stock-based compensation (Details 1) (Stock Options [Member], USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock Options [Member]
Weighted-average assumptions and resulting weighted-average grant date fair values [Abstract]
Closing price of our common stock on grant date	$ 74.56	$ 54.66	$ 58.32
Expected volatility	22.20%	23.50%	28.00%
Expected life (in years)	8 years 1 month 6 days	5 years 10 months 24 days	6 years 7 months 6 days
Risk-free interest rate	1.60%	2.50%	3.20%
Expected dividend yield	2.10%	2.00%	0.00%
Fair value of stock options granted	$ 14.65	$ 11.39	$ 20.97