|**Operating Leases (Tables)**    | 12 Months Ended                                                                                                                                                                                                                            
|-------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Leases, Operating [Abstract]** |                                                                                                                                                                                                                                            
|Future minimum lease payments    | The table below presents the expected future minimum lease payments to be made under non-cancelable operating leases as ofDecember 31, 2017.(In millions)Future Minimum Lease Payments2018$40201935202037202138202235Thereafter232Total$417
|                                
|                                 |                                                                                                                                                                                                                                            
|(In millions)                    | Future Minimum Lease Payments                                                                                                                                                                                                              
|2018                             | $                                                                                                                                                                                                                                          
|2019                             | 35                                                                                                                                                                                                                                         
|2020                             | 37                                                                                                                                                                                                                                         
|2021                             | 38                                                                                                                                                                                                                                         
|2022                             | 35                                                                                                                                                                                                                                         
|Thereafter                       | 232                                                                                                                                                                                                                                        
|Total                            | $                                                                                                                                                                                                                                          

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