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Cash and stock-based compensation plans (Details 2) (Stock Purchase Options [Member], USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Stock Purchase Options [Member]
     
Weighted average fair value per option $ 3.01 $ 2.92 $ 2.91
Weighted average expected term, in years 5 years 6 months 5 years 6 months 4 years 9 months
Expected volatility 33.35% 30.00% 37.00%
Risk-free rate   2.52% 2.32%
Risk-free rate Minimum 0.18%    
Risk-free rate Maximum 1.34%    
Expected dividend 5.30% 4.50% 5.00%