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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2018
Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 1,863 | $ 6,303
Fiduciary interest income | 18 | 46
Risk and Insurance Services Segment | EMEA |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 441 | 1,610
Risk and Insurance Services Segment | Asia Pacific |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 167 | 514
Risk and Insurance Services Segment | Latin America |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 96 | 279
Risk and Insurance Services Segment | Total International |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 704 | 2,403
Risk and Insurance Services Segment | U.S./Canada |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 926 | 2,670
Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,656 | 4,974
Defined Benefit Consulting & Administration | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 300 | 959
Investment Management & Related Services | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 225 | 683
Total Wealth | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 525 | 1,642
Health | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 415 | 1,286
Career | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 235 | 576
Marsh Insurance Group | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,630 | 5,073
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 215 | 1,184
Marsh & Guy Carpenter | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,845 | 6,257
Mercer Consulting Group | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,175 | 3,504
Oliver Wyman Group Consulting Group | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 481 | $ 1,470

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