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Restructuring Costs - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2020 | Jun. 30, 2020 | Dec. 31, 2019
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued |  | $ 20 | $ 112
ROU asset impairment loss | $ 11 |  |
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 |  | 6 |
Consulting costs and adjustments to restructuring liabilities for future rent under non-cancellable leases | Corporate |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued |  | 12 |
Jardine Lloyd Thompson Group plc |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued |  | 137 | 335
Jardine Lloyd Thompson Group plc | Acquisition related |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Restructuring and related cost, expected cost | 700 | 700 |
Restructuring and related cost, cost incurred to date | $ 137 | 137 | $ 335
Amounts accrued |  | 137 |
Jardine Lloyd Thompson Group plc | Acquisition related | Corporate |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued |  | 20 |
Jardine Lloyd Thompson Group plc | Acquisition related | Risk and Insurance Services Segment |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued |  | 100 |
Jardine Lloyd Thompson Group plc | Acquisition related | Consulting Segment |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued |  | $ 17 |

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