|**Leases (Tables)**                                             | 3 Months Ended                                                                                                                                                                                                                                                                                       
|--------------------------------------------------------------- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Leases [Abstract]**                                           |                                                                                                                                                                                                                                                                                                      
|Assets and Liabilities, Lessee                                  | The table below presents operating lease ROU assets and lease liabilities as of March 31, 2019:(In millions)March 31, 2019Operating lease ROU assets$233Operating lease liabilities308                                                                                                               
|                                                               
|                                                                |                                                                                                                                                                                                                                                                                                      
|(In millions)                                                   | March 31, 2019                                                                                                                                                                                                                                                                                       
|Operating lease ROU assets                                      | $                                                                                                                                                                                                                                                                                                    
|Operating lease liabilities                                     | 308                                                                                                                                                                                                                                                                                                  
|Maturities of Operating Lease Liabilities                       | The table below presents the maturities of operating lease liabilities as of March 31, 2019:(In millions)Operating Leases2019 (Excluding the three months ended March 31, 2019)$24202040202142202239202333Thereafter202Total lease payments380Less: Discount(72)Total operating lease liabilities$308
|                                                               
|                                                                |                                                                                                                                                                                                                                                                                                      
|(In millions)                                                   | Operating Leases                                                                                                                                                                                                                                                                                     
|2019 (Excluding the three months ended March 31, 2019)          | $                                                                                                                                                                                                                                                                                                    
|2020                                                            | 40                                                                                                                                                                                                                                                                                                   
|2021                                                            | 42                                                                                                                                                                                                                                                                                                   
|2022                                                            | 39                                                                                                                                                                                                                                                                                                   
|2023                                                            | 33                                                                                                                                                                                                                                                                                                   
|Thereafter                                                      | 202                                                                                                                                                                                                                                                                                                  
|Total lease payments                                            | 380                                                                                                                                                                                                                                                                                                  
|Less: Discount                                                  | (72                                                                                                                                                                                                                                                                                                  
|Total operating lease liabilities                               | $                                                                                                                                                                                                                                                                                                    
|Lease Terms And Discount Rates                                  | The table below presents the weighted average remaining lease term for operating leases and weighted average discount rate used in calculating operating lease right-of-use assets:Three months ended March 312019Weighted average remaining lease term11.0 yearsWeighted average discount rate3.4%  
|                                                               
|                                                                |                                                                                                                                                                                                                                                                                                      
|Three months ended March 31                                     | 2019                                                                                                                                                                                                                                                                                                 
|Weighted average remaining lease term                           | 11.0 years                                                                                                                                                                                                                                                                                           
|Weighted average discount rate                                  | 3.4                                                                                                                                                                                                                                                                                                  
|Schedule of Future Minimum Rental Payments for Operating Leases | The table below presents the expected future minimum lease payments to be made under non-cancelable operating leases as of December 31, 2018:(In millions)Future Minimum Lease Payments2019$35202039202141202238202332Thereafter200Total$385                                                         
|                                                               
|                                                                |                                                                                                                                                                                                                                                                                                      
|(In millions)                                                   | Future Minimum Lease Payments                                                                                                                                                                                                                                                                        
|2019                                                            | $                                                                                                                                                                                                                                                                                                    
|2020                                                            | 39                                                                                                                                                                                                                                                                                                   
|2021                                                            | 41                                                                                                                                                                                                                                                                                                   
|2022                                                            | 38                                                                                                                                                                                                                                                                                                   
|2023                                                            | 32                                                                                                                                                                                                                                                                                                   
|Thereafter                                                      | 200                                                                                                                                                                                                                                                                                                  
|Total                                                           | $                                                                                                                                                                                                                                                                                                    

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