|**Benefit Plans (Tables)**                    | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                        
|--------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Retirement Benefits [Abstract]**            |                                                                                                                                                                                                                                                                                                                                                                                       
|Components of net periodic cost (benefit)     | The components of net periodic pension cost (benefit) are presented in the following table.Three months ended March 31(In millions)20202019Net periodic pension cost (benefit)Interest cost on projected benefit obligation$20$25Expected return on plan assets(39)(36)Amortization of net actuarial (gain) loss1110Settlement loss1—Total net periodic pension cost (benefit)$(7)$(1)
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|                                              |                                                                                                                                                                                                                                                                                                                                                                                       
|Three months ended March 31                   |                                                                                                                                                                                                                                                                                                                                                                                       
|(In millions)                                 | 2020                                                                                                                                                                                                                                                                                                                                                                                  
|Net periodic pension cost (benefit)           |                                                                                                                                                                                                                                                                                                                                                                                       
|Interest cost on projected benefit obligation | $                                                                                                                                                                                                                                                                                                                                                                                     
|Expected return on plan assets                | (39                                                                                                                                                                                                                                                                                                                                                                                   
|Amortization of net actuarial (gain) loss     | 11                                                                                                                                                                                                                                                                                                                                                                                    
|Settlement loss                               | 1                                                                                                                                                                                                                                                                                                                                                                                     
|Total net periodic pension cost (benefit)     | $                                                                                                                                                                                                                                                                                                                                                                                     

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