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Restructuring Costs (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Costs incurred |  |  | $ 35 |  | $ 89
Global Information Technology, HR Functions and Future Rent |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Costs incurred | $ 12 |  | 35 |  |
Jardine Lloyd Thompson Group plc |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Costs incurred |  |  | 61 |  | $ 251
Jardine Lloyd Thompson Group plc | Acquisition related |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Restructuring and related cost, cost incurred to date | 647 |  | 647 |  |
Costs incurred | 19 | $ 44 | 61 | $ 181 |
Jardine Lloyd Thompson Group plc | Acquisition related | Risk and Insurance Services Segment |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Costs incurred | 11 |  | 38 |  |
Jardine Lloyd Thompson Group plc | Acquisition related | Consulting Segment |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Costs incurred | $ 9 |  | $ 21 |  |

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