Stock-based compensation (Details 2) (Stock Options [Member], USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Stock Options [Member]
Weighted-average assumptions and resulting weighted-average grant date fair values [Abstract]
Closing price of our common stock on grant date	$ 85.59000000	$ 74.56000000	$ 54.66000000
Expected volatility	23.10%	22.20%	23.50%
Expected life (in years)	8 years 1 month 6 days	8 years 1 month 6 days	5 years 10 months 24 days
Risk-free interest rate	1.70%	1.60%	2.50%
Expected dividend yield	2.20%	2.10%	2.00%
Fair value of stock options granted	$ 17.43	$ 14.65	$ 11.39