|**Benefit Plans**                             | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                      
|--------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Retirement Benefits [Abstract]**            |                                                                                                                                                                                                                                                                                                                                                                                                                     
|Benefit Plans                                 | Benefit PlansThe components of net periodic cost (benefit) are presented in the following table.Periods ended June 30Three MonthsSix Months(In millions)2017201620172016Pension cost (benefit)Interest cost on projected benefit obligation$26$28$52$56Expected return on plan assets(39)(40)(78)(80)Amortization of net actuarial loss9101819Settlement loss——2—Net periodic pension cost (benefit)$(4)$(2)$(6)$(5)
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|                                              |                                                                                                                                                                                                                                                                                                                                                                                                                     
|Periods ended June 30                         | Three Months                                                                                                                                                                                                                                                                                                                                                                                                        
|(In millions)                                 | 2017                                                                                                                                                                                                                                                                                                                                                                                                                
|Pension cost (benefit)                        |                                                                                                                                                                                                                                                                                                                                                                                                                     
|Interest cost on projected benefit obligation | $                                                                                                                                                                                                                                                                                                                                                                                                                   
|Expected return on plan assets                | (39                                                                                                                                                                                                                                                                                                                                                                                                                 
|Amortization of net actuarial loss            | 9                                                                                                                                                                                                                                                                                                                                                                                                                   
|Settlement loss                               | —                                                                                                                                                                                                                                                                                                                                                                                                                   
|Net periodic pension cost (benefit)           | $                                                                                                                                                                                                                                                                                                                                                                                                                   

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+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -URI http://asc.fasb.org/topic&trid=2235017
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