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Integration and Restructuring Costs (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Restructuring Cost and Reserve [Line Items] |  |  |  |
Restructuring actions costs | $ 40 | $ 44 | $ 28 |
Liability balance | 65 | 93 | 93 | $ 92
Payments for restructuring | 71 | $ 39 | $ 27 |
Operating Segments | Risk and Insurance Services Segment | Acquisition Related |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |
Restructuring actions costs | 11 |  |  |
Operating Segments | Consulting Segment | Acquisition Related |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |
Restructuring actions costs | 19 |  |  |
Corporate, Non-Segment | Acquisition Related |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |
Restructuring actions costs | $ 10 |  |  |

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