|**Benefit Plans (Tables)**                    | 6 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.Periods ended June 30Three MonthsSix Months(In millions)2018201720182017Net periodic pension cost (benefit)Service cost$—$—$—$—Non-service cost:Interest cost on projected benefit obligation24264752Expected return on plan assets(40)(39)(80)(78)Amortization of net actuarial loss991818Settlement loss1—52Total non-service cost(6)(4)(10)(6)Total net periodic pension cost (benefit)$(6)$(4)$(10)$(6)
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|Periods ended June 30                         | Three Months                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|(In millions)                                 | 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Net periodic pension cost (benefit)           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Service cost                                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Non-service cost:                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Interest cost on projected benefit obligation | 24                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Expected return on plan assets                | (40                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Amortization of net actuarial loss            | 9                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Settlement loss                               | 1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Total non-service cost                        | (6                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|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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