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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2022 | Sep. 30, 2021
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 4,770 | $ 4,583 | $ 15,698 | $ 14,683
Risk and Insurance Services |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,838 | 2,670 | 9,700 | 9,036
Fiduciary interest income | 40 | 4 | 57 | 12
Risk and Insurance Services | Total International |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,019 | 986 | 3,531 | 3,433
Risk and Insurance Services | EMEA |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 589 | 600 | 2,176 | 2,233
Risk and Insurance Services | Asia Pacific |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 312 | 281 | 1,015 | 902
Risk and Insurance Services | Latin America |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 118 | 105 | 340 | 298
Risk and Insurance Services | U.S./Canada |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,451 | 1,366 | 4,263 | 3,894
Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,951 | 1,925 | 6,045 | 5,690
Consulting | Wealth |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 561 | 613 | 1,775 | 1,861
Consulting | Health |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 451 | 449 | 1,562 | 1,398
Consulting | Career |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 272 | 253 | 679 | 618
Marsh & Guy Carpenter | Risk and Insurance Services |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,798 | 2,666 | 9,643 | 9,024
Marsh | Risk and Insurance Services |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,470 | 2,352 | 7,794 | 7,327
Guy Carpenter | Risk and Insurance Services |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 328 | 314 | 1,849 | 1,697
Mercer | Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,284 | 1,315 | 4,016 | 3,877
Oliver Wyman Group | Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 667 | $ 610 | $ 2,029 | $ 1,813

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