|**Net Income Per Common Share (Tables)**              | 3 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 March 31,20222021Numerator:Net income (loss)—basic and diluted$(273)$620Denominator:Basic weighted-average common shares outstanding160.4161.0Effect of share-based awards:Stock options—0.9Nonvested shares—0.6Diluted weighted-average shares160.4162.5Earnings (loss) per share:Basic$(1.70)$3.85Diluted$(1.70)$3.82Number of anti-dilutive share-based awards2.31.0
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|(In millions, except per share data)                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|Numerator:                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net income (loss)—basic and diluted                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Denominator:                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Basic weighted-average common shares outstanding      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Effect of share-based awards:                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Stock options                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Nonvested shares                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Diluted weighted-average shares                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Earnings (loss) 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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