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Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Oct. 31, 2016 | Sep. 30, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer matching contribution, percent of employees' gross pay (up to) | 4.00% |  |
Maximum annual contributions per employee, percent | 7.00% |  |
Contributions by employer |  | $ 193 |
Pension Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Estimated future employer contributions in current fiscal year |  | 63 |
United States |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized |  | $ 99 |
United States | Equity Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target asset allocation, percentage |  | 64.00% |
Actual asset allocation percentage of equity |  | 65.00% |
United States | Fixed Income Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target asset allocation, percentage |  | 36.00% |
Actual asset allocation percentage of equity |  | 35.00% |
United Kingdom |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized |  | $ 56 |
United Kingdom | Equity Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target asset allocation, percentage |  |  | 46.00%
Actual asset allocation percentage of equity |  | 47.00% |
United Kingdom | Fixed Income Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target asset allocation, percentage |  |  | 54.00%
Actual asset allocation percentage of equity |  | 53.00% |
United Kingdom | Geographic Concentration Risk | Non-U.S. Plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Concentration risk percentage |  | 81.00% |

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