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Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions |  | 3 Months Ended | 6 Months Ended
--- | --- | --- | ---
Dec. 31, 2023 | Jun. 30, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023
Pension Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Contributions by employer |  | $ 27 | $ 26 | $ 51 | $ 47
Estimated future employer contributions in current fiscal year |  | 43 |  | 43 |
United States |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined contribution plan, cost recognized |  | 48 | 44 | 97 | 88
United Kingdom |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined contribution plan, cost recognized |  | $ 39 | $ 38 | $ 91 | $ 81
United Kingdom | Defined Benefit Plan Assets Benchmark | Geographic Concentration Risk |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Concentration risk percentage | 79.00% |  |  |  |
Equity Funds | United States |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation, percentage |  | 50.00% |  | 50.00% |
Actual asset allocation percentage of equity |  | 51.00% |  | 51.00% |
Equity Funds | United Kingdom |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation, percentage |  | 14.00% |  | 14.00% |
Actual asset allocation percentage of equity |  | 14.00% |  | 14.00% |
Fixed Income Funds | United States |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation, percentage |  | 50.00% |  | 50.00% |
Actual asset allocation percentage of equity |  | 49.00% |  | 49.00% |
Fixed Income Funds | United Kingdom |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation, percentage |  | 86.00% |  | 86.00% |
Actual asset allocation percentage of equity |  | 86.00% |  | 86.00% |

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