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Supplemental Cash Flow Information (Tables)
 12 Months Ended
Dec. 31, 2014
Supplemental Cash Flow Elements [Abstract]  
Supplemental Cash Flow Information
     Year Ended December 31,  
             2014                      2013                      2012          
     (in millions)  

Interest paid on outstanding debt

    $       176          $       171          $       198      

Income tax payments

      129             245            332      

Income tax refunds

      9            23            22