|**Additional Shareholders' Equity Information (Tables)**                                                    | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|**Additional Shareholders' Equity Information [Abstract]**                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Schedule Of Activity In Shares Of Common And Treasury Stock                                                 | Activity in shares of our common stock and treasury stock was as follows:Year Ended December 31,20152014Common Stock Shares IssuedShares, Beginning of Period402,329,325399,841,717Exercise of Common Stock Options343,1451,459,490Restricted Stock Awards, Net of Forfeitures1,847,8021,028,118Public Equity Offering24,150,000—Shares Exchanged in Rosetta Merger41,048,240—Shares, End of Period469,718,512402,329,325Treasury StockShares, Beginning of Period37,635,89037,600,051Shares Received From Employees in Payment of Withholding Taxes Due on Vesting of Shares of Restricted Stock490,744254,888Rabbi Trust Shares Distributed and/or Sold(201,009)(219,049)Shares, End of Period37,925,62537,635,890
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|Common Stock Shares Issued                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Shares, Beginning of Period                                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Exercise of Common Stock Options                                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Restricted Stock Awards, Net of Forfeitures                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Public Equity Offering                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Shares Exchanged in Rosetta Merger                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Shares, End of Period                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Treasury Stock                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Shares, Beginning of Period                                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Shares Received From Employees in Payment of Withholding Taxes Due on Vesting of Shares of Restricted Stock |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Rabbi Trust Shares Distributed and/or Sold                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Shares, End of Period                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Accumulated other comprehensive income (loss) in the shareholders' equity section of the balance sheet      | Accumulated other comprehensive loss in the shareholders’ equity section of the balance sheet included:Accumulated Other Comprehensive Loss(millions)Interest RateCash FlowHedgesPension-Related andOtherTotalDecember 31, 2012$(25)$(88)$(113)Realized Amounts Reclassified Into Earnings11213Unrealized Change in Fair Value—(17)(17)December 31, 2013(24)(93)(117)Realized Amounts Reclassified Into Earnings11112Unrealized Change in Fair Value—1515December 31, 2014(23)(67)(90)Realized Amounts Reclassified Into Earnings16263Unrealized Change in Fair Value—(6)(6)December 31, 2015$(22)$(11)$(33)                                                                                                        
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|                                                                                                            | Accumulated Other Comprehensive Loss                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|(millions)                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|December 31, 2012                                                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Realized Amounts Reclassified Into Earnings                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Unrealized Change in Fair Value                                                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|December 31, 2013                                                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Realized Amounts Reclassified Into Earnings                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Unrealized Change in Fair Value                                                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|December 31, 2014                                                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Realized Amounts Reclassified Into Earnings                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Unrealized Change in Fair Value                                                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|December 31, 2015                                                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     

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