|**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 31,201920182017Numerator:Net income—basic and diluted$1,997$287$1,045Denominator:Basic weighted-average common shares outstanding163.2163.2164.2Effect of share-based awards:Stock options1.20.81.1Nonvested shares0.70.50.7Diluted weighted-average shares165.1164.5166.0Earnings per share:Basic$12.24$1.76$6.36Diluted12.101.756.29Number of anti-dilutive share-based awards—1.30.7
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|(In millions, except per share data)                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|                                                      | 2019                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|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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X- ReferencesNo definition available.
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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.
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