|**Debt (Tables)**                           | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
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|**Debt [Abstract]**                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Schedule of debt                            | Debt consists of the following:March 31, 2015December 31, 2014(millions, except percentages)DebtInterest RateDebtInterest RateCredit Facility, due October 3, 2018$——%$——%Capital Lease and Other Obligations419—%413—%8.25% Senior Notes, due March 1, 20191,0008.25%1,0008.25%4.15% Senior Notes, due December 15, 20211,0004.15%1,0004.15%7.25% Senior Notes, due October 15, 20231007.25%1007.25%3.90% Senior Notes, due November 15, 20246503.90%6503.90%8.00% Senior Notes, due April 1, 20272508.00%2508.00%6.00% Senior Notes, due March 1, 20418506.00%8506.00%5.25% Senior Notes, due November 15, 20431,0005.25%1,0005.25%5.05% Senior Notes, due November 15, 20448505.05%8505.05%7.25% Senior Debentures, due August 1, 2097847.25%847.25%Total6,2036,197Unamortized Discount(25)(26)Total Debt, Net of Discount6,1786,171Less Amounts Due Within One YearCapital Lease Obligations(65)(68)Long-Term Debt Due After One Year$6,113$6,103
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|                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|                                            | March 31, 2015                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|(millions, except percentages)              | Debt                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Credit Facility, due October 3, 2018        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Capital Lease and Other Obligations         | 419                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|8.25% Senior Notes, due March 1, 2019       | 1,000                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|4.15% Senior Notes, due December 15, 2021   | 1,000                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|7.25% Senior Notes, due October 15, 2023    | 100                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|3.90% Senior Notes, due November 15, 2024   | 650                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|8.00% Senior Notes, due April 1, 2027       | 250                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|6.00% Senior Notes, due March 1, 2041       | 850                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|5.25% Senior Notes, due November 15, 2043   | 1,000                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|5.05% Senior Notes, due November 15, 2044   | 850                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|7.25% Senior Debentures, due August 1, 2097 | 84                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Total                                       | 6,203                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Unamortized Discount                        | (25                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Total Debt, Net of Discount                 | 6,178                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Less Amounts Due Within One Year            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Capital Lease Obligations                   | (65                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Long-Term Debt Due After One Year           | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    

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