|**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 30,2016201520162015Numerator:Net income—basic and diluted$180$174$491$478Denominator:Basic weighted-average common shares outstanding164.6164.0164.5164.1Effect of share-based awards:Stock options1.21.01.10.9Nonvested shares1.00.50.90.5Diluted weighted-average shares166.8165.5166.5165.5Earnings per share:Basic$1.09$1.06$2.98$2.91Diluted1.081.052.952.89Number of anti-dilutive share-based awards:—0.30.30.7
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|(In millions except per share data)                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|                                                      | 2016                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|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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