|**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)20192018Net periodic pension cost (benefit)Service cost$—$—Non-service cost (benefit):Interest cost on projected benefit obligation2523Expected return on plan assets(36)(40)Amortization of net actuarial (gain) loss109Settlement loss—4Total non-service cost (benefit)(1)(4)Total net periodic pension cost (benefit)$(1)$(4)
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|Three months ended March 31                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|(In millions)                                 | 2019                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Net periodic pension cost (benefit)           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Service cost                                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Non-service cost (benefit):                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Interest cost on projected benefit obligation | 25                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Expected return on plan assets                | (36                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Amortization of net actuarial (gain) loss     | 10                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Settlement loss                               | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Total non-service cost (benefit)              | (1                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Total net periodic pension cost (benefit)     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

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duration X- DefinitionTabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments.
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