|**Leases (Tables)**                                             | 6 Months Ended                                                                                                                                                                                                                                                                                      
|--------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Leases [Abstract]**                                           |                                                                                                                                                                                                                                                                                                     
|Assets and Liabilities, Lessee                                  | The following table presents operating lease ROU assets and lease liabilities.(In millions)June 30, 2019Operating lease ROU assets$223Operating lease liabilities301                                                                                                                                
|                                                               
|                                                                |                                                                                                                                                                                                                                                                                                     
|(In millions)                                                   | June 30, 2019                                                                                                                                                                                                                                                                                       
|Operating lease ROU assets                                      | $                                                                                                                                                                                                                                                                                                   
|Operating lease liabilities                                     | 301                                                                                                                                                                                                                                                                                                 
|Maturities of Operating Lease Liabilities                       | The following table presents the maturities of operating lease liabilities as ofJune 30, 2019.(In millions)Operating Leases2019 (Excluding the six months ended June 30, 2019)$15202039202142202238202333Thereafter202Total lease payments369Less: Discount(68)Total operating lease liabilities$301
|                                                               
|                                                                |                                                                                                                                                                                                                                                                                                     
|(In millions)                                                   | Operating Leases                                                                                                                                                                                                                                                                                    
|2019 (Excluding the six months ended June 30, 2019)             | $                                                                                                                                                                                                                                                                                                   
|2020                                                            | 39                                                                                                                                                                                                                                                                                                  
|2021                                                            | 42                                                                                                                                                                                                                                                                                                  
|2022                                                            | 38                                                                                                                                                                                                                                                                                                  
|2023                                                            | 33                                                                                                                                                                                                                                                                                                  
|Thereafter                                                      | 202                                                                                                                                                                                                                                                                                                 
|Total lease payments                                            | 369                                                                                                                                                                                                                                                                                                 
|Less: Discount                                                  | (68                                                                                                                                                                                                                                                                                                 
|Total operating lease liabilities                               | $                                                                                                                                                                                                                                                                                                   
|Lease Terms and Discount Rates                                  | The following table presents the weighted average remaining lease term for operating leases and weighted average discount rate used in calculating operating lease right-of-use assets.June 30, 2019Weighted average remaining lease term10.8 yearsWeighted average discount rate3.4%               
|                                                               
|                                                                |                                                                                                                                                                                                                                                                                                     
|                                                                | June 30, 2019                                                                                                                                                                                                                                                                                       
|Weighted average remaining lease term                           | 10.8 years                                                                                                                                                                                                                                                                                          
|Weighted average discount rate                                  | 3.4                                                                                                                                                                                                                                                                                                 
|Schedule of Future Minimum Rental Payments for Operating Leases | The following table presents the expected future minimum lease payments to be made under non-cancelable operating leases as ofDecember 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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