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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 5,017 | $ 4,189 | $ 10,100 | $ 8,840
Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 3,141 | 2,603 | 6,366 | 5,514
Fiduciary interest income | 3 | 9 | 8 | 32
Risk and Insurance Services Segment | Total International |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,246 | 994 | 2,447 | 2,077
Risk and Insurance Services Segment | EMEA |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 796 | 597 | 1,633 | 1,351
Risk and Insurance Services Segment | Asia Pacific |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 347 | 298 | 621 | 536
Risk and Insurance Services Segment | Latin America |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 103 | 99 | 193 | 190
Risk and Insurance Services Segment | U.S./Canada |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,404 | 1,167 | 2,528 | 2,145
Consulting Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,892 | 1,616 | 3,765 | 3,378
Consulting Segment | Wealth |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 625 | 561 | 1,248 | 1,153
Consulting Segment | Health |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 462 | 432 | 949 | 918
Consulting Segment | Career |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 187 | 156 | 365 | 329
Marsh & Guy Carpenter | Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 3,138 | 2,594 | 6,358 | 5,482
Marsh Insurance Group | Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,650 | 2,161 | 4,975 | 4,222
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 488 | 433 | 1,383 | 1,260
Mercer Consulting Group | Consulting Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,274 | 1,149 | 2,562 | 2,400
Oliver Wyman Group Consulting Group | Consulting Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 618 | $ 467 | $ 1,203 | $ 978

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