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Restructuring Costs - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2019 | Dec. 31, 2018
Restructuring Cost and Reserve [Line Items] |  |  |
Restructuring And related cost, period for expected cost |  | 3 years |
Amounts accrued |  | $ 56 | $ 161
Jardine Lloyd Thompson Group plc |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued |  | 192 |
Jardine Lloyd Thompson Group plc | Acquisition related |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Restructuring and related cost, expected cost | $ 375 | 375 |
Restructuring and related cost, cost incurred to date | 192 | 192 |
Amounts accrued | 77 |  |
Risk and Insurance Services Segment | Severance and consulting costs |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued |  | 6 |
Risk and Insurance Services Segment | Jardine Lloyd Thompson Group plc | Acquisition related |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued | 58 | 134 |
Consulting Segment | Jardine Lloyd Thompson Group plc | Acquisition related |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued | 5 | 10 |
Corporate | Severance and future rent under non-cancelable leases and other costs |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued |  | 7 |
Corporate | Jardine Lloyd Thompson Group plc | Acquisition related |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued | 14 | 48 |
Mercer Consulting Group | Severance and consulting costs |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Amounts accrued | $ 10 | $ 43 |

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