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Integration and Restructuring Costs (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Restructuring actions costs |  | $ 161 | $ 40 | $ 44 |
Liability balance | $ 112 | 112 | 65 | 93 | $ 93
Payments for restructuring |  | 114 | 71 | 44 |
Severance |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Restructuring costs | 51 | 96 |  |  |
Restructuring actions costs |  | 137 | 31 |  |
Liability balance | $ 73 | 73 | 15 | $ 32 |
Payments for restructuring |  | 77 | $ 49 |  |
Severance and Future Rent |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Restructuring actions costs |  | 14 |  |  |
Operating Segments | Risk and Insurance Services Segment | Acquisition Related |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Restructuring actions costs |  | 3 |  |  |
Operating Segments | Consulting Segment | Acquisition Related |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Restructuring actions costs |  | 1 |  |  |
Corporate, Non-Segment | Acquisition Related |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Restructuring actions costs |  | $ 10 |  |  |

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