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Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 4,293 $ 3,813 $ 3,653
Alternative Investments, Fair Value Disclosure 2,213 1,995  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 566 494  
Pension Benefits | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1,640 1,434  
Pension Benefits | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Cash And Cash Equivalents      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 25 33  
Alternative Investments, Fair Value Disclosure 25 33  
Pension Benefits | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Cash And Cash Equivalents | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | U.S. large capitalization      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1,523 1,352  
Alternative Investments, Fair Value Disclosure 1,523 1,352  
Pension Benefits | U.S. large capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | U.S. large capitalization | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | U.S. small and mid-capitalization      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 379 361  
Alternative Investments, Fair Value Disclosure 0 0  
Pension Benefits | U.S. small and mid-capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 379 361  
Pension Benefits | U.S. small and mid-capitalization | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | U.S. small and mid-capitalization | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | International and emerging markets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 629 522  
Alternative Investments, Fair Value Disclosure 450 389  
Pension Benefits | International and emerging markets | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 179 133  
Pension Benefits | International and emerging markets | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | International and emerging markets | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Corporate Bonds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 741 630  
Alternative Investments, Fair Value Disclosure 15 13  
Pension Benefits | Corporate Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 726 617  
Pension Benefits | Corporate Bonds | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Municipal Bonds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 91 95  
Alternative Investments, Fair Value Disclosure 0 0  
Pension Benefits | Municipal Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 91 95  
Pension Benefits | Municipal Bonds | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | U.S. treasury and agency securities      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 824 701  
Alternative Investments, Fair Value Disclosure 0 0  
Pension Benefits | U.S. treasury and agency securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 8 0  
Pension Benefits | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 816 701  
Pension Benefits | U.S. treasury and agency securities | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Other      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 7 21  
Alternative Investments, Fair Value Disclosure 0 0  
Pension Benefits | Other | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Other | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 7 21  
Pension Benefits | Other | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Real estate      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 196 202  
Alternative Investments, Fair Value Disclosure 196 202  
Pension Benefits | Real estate | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Real estate | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Real estate | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Private equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 4 6  
Alternative Investments, Fair Value Disclosure 4 6  
Pension Benefits | Private equity | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Private equity | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Private equity | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Pension Benefits | Medical benefit assets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets (153) (132)  
Pension Benefits | Net receivables      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 27 22  
Pension Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Receivables Related To Pending Security Sales [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 4,419 3,923  
Pension Benefits | Excludes Medical Benefit Component Under Section401 H And Includes Receivables Related To Pending Security Sales [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 4,293 3,813  
Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1,223 1,101 $ 1,071
Alternative Investments, Fair Value Disclosure 245 212  
Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 509 456  
Postretirement Benefits | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 342 326  
Postretirement Benefits | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Cash And Cash Equivalents      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 44 53  
Alternative Investments, Fair Value Disclosure 0 0  
Postretirement Benefits | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 44 53  
Postretirement Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Cash And Cash Equivalents | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | U.S. large capitalization      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 442 392  
Alternative Investments, Fair Value Disclosure 110 101  
Postretirement Benefits | U.S. large capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 332 291  
Postretirement Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | U.S. large capitalization | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | U.S. small and mid-capitalization      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 80 72  
Alternative Investments, Fair Value Disclosure 0 0  
Postretirement Benefits | U.S. small and mid-capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 80 72  
Postretirement Benefits | U.S. small and mid-capitalization | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | U.S. small and mid-capitalization | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | International and emerging markets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 154 132  
Alternative Investments, Fair Value Disclosure 101 92  
Postretirement Benefits | International and emerging markets | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 53 40  
Postretirement Benefits | International and emerging markets | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | International and emerging markets | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Other Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 8 7  
Alternative Investments, Fair Value Disclosure 0 0  
Postretirement Benefits | Other Equity | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Other Equity | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 8 7  
Postretirement Benefits | Other Equity | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Corporate Bonds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 144 141  
Alternative Investments, Fair Value Disclosure 0 0  
Postretirement Benefits | Corporate Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 144 141  
Postretirement Benefits | Corporate Bonds | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Municipal Bonds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 110 110  
Alternative Investments, Fair Value Disclosure 0 0  
Postretirement Benefits | Municipal Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 110 110  
Postretirement Benefits | Municipal Bonds | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | U.S. treasury and agency securities      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 76 68  
Alternative Investments, Fair Value Disclosure 0 0  
Postretirement Benefits | U.S. treasury and agency securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 76 68  
Postretirement Benefits | U.S. treasury and agency securities | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Other      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 38 19  
Alternative Investments, Fair Value Disclosure 34 19  
Postretirement Benefits | Other | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Other | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 4 0  
Postretirement Benefits | Other | Significant Other Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0 0  
Postretirement Benefits | Medical benefit assets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 153 132  
Postretirement Benefits | Net payables      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets (26) (25)  
Postretirement Benefits | Excludes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1,096 994  
Postretirement Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 1,223 $ 1,101