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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 4,583 | $ 3,968 | $ 14,683 | $ 12,808
Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,670 | 2,291 | 9,036 | 7,805
Fiduciary interest income | 4 | 8 | 12 | 40
Risk and Insurance Services Segment | Total International |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 986 | 883 | 3,433 | 2,960
Risk and Insurance Services Segment | EMEA |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 600 | 536 | 2,233 | 1,887
Risk and Insurance Services Segment | Asia Pacific |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 281 | 254 | 902 | 790
Risk and Insurance Services Segment | Latin America |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 105 | 93 | 298 | 283
Risk and Insurance Services Segment | U.S./Canada |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,366 | 1,126 | 3,894 | 3,271
Consulting Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,925 | 1,696 | 5,690 | 5,074
Consulting Segment | Wealth |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 613 | 566 | 1,861 | 1,719
Consulting Segment | Health |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 449 | 430 | 1,398 | 1,348
Consulting Segment | Career |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 253 | 220 | 618 | 549
Marsh & Guy Carpenter | Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,666 | 2,283 | 9,024 | 7,765
Marsh Insurance Group | Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,352 | 2,009 | 7,327 | 6,231
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 314 | 274 | 1,697 | 1,534
Mercer Consulting Group | Consulting Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,315 | 1,216 | 3,877 | 3,616
Oliver Wyman Group Consulting Group | Consulting Segment |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 610 | $ 480 | $ 1,813 | $ 1,458

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