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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Risk and Insurance Services Segment |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | $ 8,228 | $ 7,630 |
Interest income, fiduciary assets | 65 | 39 | $ 26
Risk and Insurance Services Segment | EMEA |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,132 | 2,033 |
Risk and Insurance Services Segment | Asia Pacific |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 683 | 645 |
Risk and Insurance Services Segment | Latin America |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 400 | 404 |
Risk and Insurance Services Segment | International |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 3,215 | 3,082 |
Risk and Insurance Services Segment | Americas |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 3,662 | 3,322 |
Consulting Segment |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 6,779 | 6,444 |
Interest income, fiduciary assets | 3 | 4 | $ 3
Consulting Segment | Defined Benefit Consulting & Administration |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 1,279 | 1,381 |
Consulting Segment | Investment Management & Related Services |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 906 | 767 |
Consulting Segment | Total Wealth |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,185 | 2,148 |
Consulting Segment | Health |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 1,735 | 1,648 |
Consulting Segment | Career |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 812 | 732 |
Marsh Insurance Group | Risk and Insurance Services Segment |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 6,877 | 6,404 |
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 1,286 | 1,187 |
Marsh & Guy Carpenter | Risk and Insurance Services Segment |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 8,163 | 7,591 |
Mercer Consulting Group | Consulting Segment |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 4,732 | 4,528 |
Oliver Wyman Group Consulting Group | Consulting Segment |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | $ 2,047 | $ 1,916 |

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