|**Benefit Plans**                                     | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|----------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Compensation and Retirement Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Benefit Plans                                         | NoteG. Benefit PlansThe components of net periodic cost (benefit) are presented in the following table.Three months ended March 31(In millions)20152014Pension cost (benefit)Service cost$2$3Interest cost on projected benefit obligation2833Expected return on plan assets(43)(48)Amortization of net actuarial loss96Net periodic pension cost (benefit)$(4)$(6)Postretirement cost (benefit)Amortization of prior service credit$—$(4)Net periodic postretirement cost (benefit)$—$(4)
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|Three months ended March 31                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|(In millions)                                         | 2015                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Pension cost (benefit)                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Service cost                                          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Interest cost on projected benefit obligation         | 28                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Expected return on plan assets                        | (43                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Amortization of net actuarial loss                    | 9                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Net periodic pension cost (benefit)                   | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|Postretirement cost (benefit)                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Amortization of prior service credit                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Net periodic postretirement cost (benefit)            | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         

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