|**Net Income Per Common Share (Tables)**              | 9 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 September 30,Nine months ended September 30,2020201920202019Numerator:Net income—basic and diluted$484$248$167$1,371Denominator:Basic weighted-average common shares outstanding160.9163.3161.3163.2Effect of share-based awards:Stock options0.51.40.71.2Nonvested shares0.60.90.50.7Diluted weighted-average shares162.0165.6162.5165.1Earnings per share:Basic$3.01$1.51$1.03$8.40Diluted$2.99$1.49$1.03$8.30Number of anti-dilutive share-based awards1.4—1.40.4
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|(In millions, except per share data)                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
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|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.
+ ReferencesReference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257
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