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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2018
Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 2,096 | $ 4,440
Fiduciary interest income | 15 | 28
Risk and Insurance Services Segment | EMEA |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 526 | 1,169
Risk and Insurance Services Segment | Asia Pacific |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 183 | 347
Risk and Insurance Services Segment | Latin America |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 99 | 183
Risk and Insurance Services Segment | Total International |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 808 | 1,699
Risk and Insurance Services Segment | U.S./Canada |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 941 | 1,744
Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,650 | 3,318
Defined Benefit Consulting & Administration | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 320 | 659
Investment Management & Related Services | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 232 | 458
Total Wealth | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 552 | 1,117
Health | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 429 |
Career | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 177 |
Marsh Insurance Group | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,749 | 3,443
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 332 | 969
Marsh & Guy Carpenter | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 2,081 | 4,412
Mercer Consulting Group | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,158 | 2,329
Oliver Wyman Group Consulting Group | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 492 | $ 989

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