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Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items] |  |
Contributions by employer | $ 33 |
Pension Benefits |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated future employer contributions in current fiscal year | 76 |
United States |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined contribution plan, cost recognized | $ 34 | $ 34
United States | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocation, percentage | 64.00% |
Actual asset allocation percentage of equity | 63.00% |
United States | Fixed Income Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocation, percentage | 36.00% |
Actual asset allocation percentage of equity | 37.00% |
United Kingdom |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined contribution plan, cost recognized | $ 22 | $ 20
United Kingdom | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocation, percentage | 34.00% |
Actual asset allocation percentage of equity | 37.00% |
United Kingdom | Fixed Income Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocation, percentage | 66.00% |
Actual asset allocation percentage of equity | 63.00% |
United Kingdom | Geographic Concentration Risk | Non-U.S. Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Concentration risk percentage | 81.00% |

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