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Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension, post-retirement and post-employment benefits | $ 2,248 |  | $ 2,336
Pension Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions by employer | 27 |  |
Estimated future employer contributions in current fiscal year | 87 |  |
Jardine Lloyd Thompson Group plc |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension, post-retirement and post-employment benefits |  |  | 255
Pension, post-retirement and post-employment liabilities |  |  | 1,003
Pension, post-retirement and post-employment plan assets |  |  | $ 748
United States |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized | 37 | $ 35 |
United States | Pension Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Emploer contributions deferred | 47 |  |
United Kingdom |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized | $ 31 | $ 22 |
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 | 60.00% |  |
Equity Funds | United Kingdom |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target asset allocation, percentage | 33.00% |  |
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 | 40.00% |  |
Fixed Income Funds | United Kingdom |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target asset allocation, percentage | 67.00% |  |
Actual asset allocation percentage of equity | 70.00% |  |

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