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Retirement Plans (Tables)
9 Months Ended
Sep. 30, 2011
Compensation and Retirement Disclosure [Abstract] 
Components of net periodic pension cost
                                 
    Three Months     Nine Months  
    Ended September 30,     Ended September 30,  
    2011     2010     2011     2010  
            (Millions of dollars)          
Service cost
  $ 15     $ 12     $ 43     $ 36  
Interest cost
    23       22       67       66  
Expected return on plan assets
    (28 )     (21 )     (82 )     (63 )
Amortization of net loss
    13       12       35       36  
 
                       
Pension expense
  $ 23     $ 25     $ 63     $ 75