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Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018 | Mar. 31, 2019
Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Contributions by employer | $ 85 |  |  |
Estimated future employer contributions in current fiscal year | 32 |  |  |
Jardine Lloyd Thompson Group plc |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension, post-retirement and post-employment liabilities |  |  |  | $ 248
Pension, post-retirement and post-employment plan assets |  |  |  | $ 700
United States |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined contribution plan, cost recognized | $ 106 | $ 100 |  |
United States | Equity Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target asset allocation, percentage | 64.00% |  |  |
Actual asset allocation percentage of equity | 62.00% |  |  |
United States | Fixed Income Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target asset allocation, percentage | 36.00% |  |  |
Actual asset allocation percentage of equity | 38.00% |  |  |
United Kingdom |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined contribution plan, cost recognized | $ 74 | $ 61 |  |
United Kingdom | Equity Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target asset allocation, percentage | 36.00% |  |  |
Actual asset allocation percentage of equity | 33.00% |  |  |
United Kingdom | Fixed Income Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target asset allocation, percentage | 64.00% |  |  |
Actual asset allocation percentage of equity | 67.00% |  |  |
United Kingdom | Geographic Concentration Risk | Non-U.S. Plans |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Concentration risk percentage |  |  | 81.00% |

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