|**Net Income Per Common Share (Tables)**              | 6 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)Three months ended June 30,Six months ended June 30,2019201820192018Numerator:Net income—basic and diluted$428$217$1,123$186Denominator:Basic weighted-average common shares outstanding163.3163.2163.1163.6Effect of share-based awards:Stock options1.20.81.10.9Nonvested shares0.70.50.70.5Diluted weighted-average shares165.2164.5164.9165.0Earnings per share:Basic$2.62$1.33$6.89$1.13Diluted$2.59$1.32$6.81$1.12Number of anti-dilutive share-based awards0.41.30.71.3
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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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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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