|**Basic and Diluted Earnings Per Share (Tables)**                                                                                          | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
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|**Earnings Per Share [Abstract]**                                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Calculation of Basic and Diluted Earnings (Loss) Per Share                                                                                 | The following table summarizes the calculation of basic and diluted earnings per share:Three Months Ended June 30,Six  Months Ended June 30,2012201120122011(millions, except per share amounts)Income from Continuing Operations$275$269$524$235Earnings Adjustment from Assumed Conversion of Dilutive Shares of Common Stock in Rabbi Trust(1)(7)(4)(5)—Income from Continuing Operations Used for Diluted Earnings Per Share Calculation$268$265$519$235Weighted Average Number of Shares Outstanding, Basic178176178176Incremental Shares From Assumed Conversion of Dilutive Stock Options, Restricted Stock and Shares of Common Stock in Rabbi Trust2322Weighted Average Number of Shares Outstanding, Diluted180179180178Earnings from Continuing Operations Per Share, Basic$1.55$1.51$2.95$1.33Earnings from Continuing Operations Per Share, Diluted1.491.472.881.31Number of antidilutive stock options, shares of restricted stock and shares of common stock in rabbi trust excluded from calculation above3233(1)Consistent with GAAP, when dilutive, deferred compensation gains or losses, net of tax, are excluded from net income while our common shares held in the rabbi trust are included in the diluted share count. For this reason, the diluted earnings per share calculations for the three months ended June 30,  2012 and 2011 and for the six months ended June 30, 2012 exclude deferred compensation gains, net of tax.
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|                                                                                                                                           | Three Months Ended June 30,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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|(millions, except per share amounts)                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Income from Continuing Operations                                                                                                          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Earnings Adjustment from Assumed Conversion of Dilutive Shares of Common Stock in Rabbi Trust(1)                                           | (7                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Income from Continuing Operations Used for Diluted Earnings Per Share Calculation                                                          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|Weighted Average Number of Shares Outstanding, Basic                                                                                       | 178                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Incremental Shares From Assumed Conversion of Dilutive Stock Options, Restricted Stock and Shares of Common Stock in Rabbi Trust           | 2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Weighted Average Number of Shares Outstanding, Diluted                                                                                     | 180                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Earnings from Continuing Operations Per Share, Basic                                                                                       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Earnings from Continuing Operations Per Share, Diluted                                                                                     | 1.49                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Number of antidilutive stock options, shares of restricted stock and shares of common stock in rabbi trust excluded from calculation above | 3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|(1)                                                                                                                                        | Consistent with GAAP, when dilutive, deferred compensation gains or losses, net of tax, are excluded from net income while our common shares held in the rabbi trust are included in the diluted share count. For this reason, the diluted earnings per share calculations for the three months ended June 30,  2012 and 2011 and for the six months ended June 30, 2012 exclude deferred compensation gains, net of tax.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 

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