|**Employee Separation Charges (Tables)**            | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                 
|--------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Restructuring and Related Activities [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                
|Summary of separation activity                      | The following table summarizes the 2015 and 2016 separation activity:(Millions of dollars)Liability balance at December 31, 2014$—Increase in liability (separation charges)15Reduction in liability (payments)(5)Liability balance at December 31, 2015$10Increase in liability (separation charges)—Reduction in liability (payments)(10)Liability balance at June 30, 2016$—
|                                                   
|                                                    |                                                                                                                                                                                                                                                                                                                                                                                
|(Millions of dollars)                               |                                                                                                                                                                                                                                                                                                                                                                                
|Liability balance at December 31, 2014              | $                                                                                                                                                                                                                                                                                                                                                                              
|Increase in liability (separation charges)          | 15                                                                                                                                                                                                                                                                                                                                                                             
|Reduction in liability (payments)                   | (5                                                                                                                                                                                                                                                                                                                                                                             
|Liability balance at December 31, 2015              | $                                                                                                                                                                                                                                                                                                                                                                              
|Increase in liability (separation charges)          | —                                                                                                                                                                                                                                                                                                                                                                              
|Reduction in liability (payments)                   | (10                                                                                                                                                                                                                                                                                                                                                                            
|Liability balance at June 30, 2016                  | $                                                                                                                                                                                                                                                                                                                                                                              
|                                                    |                                                                                                                                                                                                                                                                                                                                                                                

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