Income per Share (Tables)
 6 Months Ended
Jun. 30, 2011
Earnings Per Share [Abstract]  
Reconciliation of the number of shares used for the basic and diluted income per share calculations
 
Three Months Ended
  
Six Months Ended
 
   
June 30
  
June 30
 
Millions of shares
 
2011
  
2010
  
2011
  
2010
 
Basic weighted average common shares outstanding
      916       906       915       906 
Dilutive effect of stock options
          5           3           5           2 
Diluted weighted average common shares outstanding
      921       909       920       908