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Restructuring Costs (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Restructuring Cost and Reserve [Line Items] |  |  |
Costs incurred | $ 11 |  | $ 89
Severance and Future Rent |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Costs incurred | 11 |  |
Jardine Lloyd Thompson Group plc |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Costs incurred | 23 |  | $ 251
Jardine Lloyd Thompson Group plc | Acquisition related |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Restructuring and related cost, cost incurred to date | 609 |  |
Costs incurred | 23 | $ 80 |
Jardine Lloyd Thompson Group plc | Acquisition related | Corporate |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Costs incurred | 1 |  |
Jardine Lloyd Thompson Group plc | Acquisition related | Risk and Insurance Services Segment |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Costs incurred | 16 |  |
Jardine Lloyd Thompson Group plc | Acquisition related | Consulting Segment |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Costs incurred | $ 6 |  |

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