|**Net Income Per Common Share (Tables)**              | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|**Earnings Per Share [Abstract]**                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Calculations For Basic And Diluted Earnings Per Share | The table shows calculations for basic and diluted earnings per share:(In millions, except per share data)Years ended December 31,201820172016Numerator:Net income—basic and diluted$287$1,045$591Denominator:Basic weighted-average common shares outstanding163.2164.2164.5Effect of share-based awards:Stock options0.81.11.1Nonvested shares0.50.70.9Diluted weighted-average shares164.5166.0166.5Earnings per share:Basic$1.76$6.36$3.59Diluted1.756.293.55Number of anti-dilutive share-based awards1.30.70.3
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|(In millions, except per share data)                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|                                                      | 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Numerator:                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net income—basic and diluted                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Denominator:                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Basic weighted-average common shares outstanding      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Effect of share-based awards:                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Stock options                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Nonvested shares                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Diluted weighted-average shares                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Earnings per share:                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Basic                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Diluted                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Number of anti-dilutive share-based awards            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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duration X- DefinitionTabular disclosure of an entity's basic and diluted earnings per share calculations, including a reconciliation of numerators and denominators of the basic and diluted per-share computations for income from continuing operations.
+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257
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