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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 4,189 | $ 4,349 | $ 8,840 | $ 8,420
Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,603 | 2,574 | 5,514 | 4,997
Fiduciary interest income | 9 | 26 | 32 | 49
Risk and Insurance Services Segment | EMEA |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 597 | 652 | 1,351 | 1,285
Risk and Insurance Services Segment | Asia Pacific |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 298 | 291 | 536 | 456
Risk and Insurance Services Segment | Latin America |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 99 | 116 | 190 | 194
Risk and Insurance Services Segment | Total International |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 994 | 1,059 | 2,077 | 1,935
Risk and Insurance Services Segment | U.S./Canada |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,167 | 1,097 | 2,145 | 1,958
Consulting Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,616 | 1,800 | 3,378 | 3,473
Consulting Segment | Wealth |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 561 | 613 | 1,153 | 1,156
Consulting Segment | Health |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 432 | 458 | 918 | 900
Consulting Segment | Career |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 156 | 189 | 329 | 359
Marsh Insurance Group | Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,161 | 2,156 | 4,222 | 3,893
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 433 | 392 | 1,260 | 1,055
Marsh & Guy Carpenter | Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,594 | 2,548 | 5,482 | 4,948
Mercer Consulting Group | Consulting Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,149 | 1,260 | 2,400 | 2,415
Oliver Wyman Group Consulting Group | Consulting Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 467 | $ 540 | $ 978 | $ 1,058

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