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Restructuring Costs - Narrative (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2020 | Dec. 31, 2019
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | $ 43 | $ 112
ROU asset impairment loss | 3 |
Severance and consulting costs | Marsh Insurance Group |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 2 |
Severance and consulting costs | Mercer Consulting Group |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 17 |
Cost To Modernize IT Systems And Security Protocols, Consulting Costs, And Adjustments To Restructuring Liabilities | Corporate |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 24 |
Jardine Lloyd Thompson Group plc |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 181 | 335
ROU asset impairment loss | 9 |
Jardine Lloyd Thompson Group plc | Acquisition related |  |
Restructuring Cost and Reserve [Line Items] |  |
Restructuring and related cost, expected cost | 700 |
Restructuring and related cost, cost incurred to date | 181 | $ 335
Amounts accrued | 181 |
Jardine Lloyd Thompson Group plc | Acquisition related | Corporate |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 25 |
Jardine Lloyd Thompson Group plc | Acquisition related | Risk and Insurance Services Segment |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 125 |
Jardine Lloyd Thompson Group plc | Acquisition related | Consulting Segment |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | $ 31 |

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