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Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions |  | 3 Months Ended
--- | --- | ---
Dec. 31, 2019 | Mar. 31, 2021 | Mar. 31, 2020
Pension Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions by employer |  | $ 29 |
Estimated future employer contributions in current fiscal year |  | 100 |
United States |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized |  | 39 | $ 37
United Kingdom |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized |  | $ 39 | $ 31
United Kingdom | Non-U.S. Plans | Geographic Concentration Risk |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Concentration risk percentage | 81.00% |  |
Equity Funds | United States |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target asset allocation, percentage |  | 64.00% |
Actual asset allocation percentage of equity |  | 65.00% |
Equity Funds | United Kingdom |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target asset allocation, percentage |  | 28.00% |
Actual asset allocation percentage of equity |  | 30.00% |
Fixed Income Funds | United States |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target asset allocation, percentage |  | 36.00% |
Actual asset allocation percentage of equity |  | 35.00% |
Fixed Income Funds | United Kingdom |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target asset allocation, percentage |  | 72.00% |
Actual asset allocation percentage of equity |  | 70.00% |

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