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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 5,083 | $ 4,651
Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 3,225 | 2,911
Fiduciary interest income | 5 | 23
Risk and Insurance Services Segment | Total International |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,201 | 1,083
Risk and Insurance Services Segment | EMEA |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 837 | 754
Risk and Insurance Services Segment | Asia Pacific |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 274 | 238
Risk and Insurance Services Segment | Latin America |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 90 | 91
Risk and Insurance Services Segment | U.S./Canada |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,124 | 978
Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,873 | 1,762
Consulting Segment | Wealth |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 623 | 592
Consulting Segment | Health |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 487 | 486
Consulting Segment | Career |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 178 | 173
Marsh & Guy Carpenter | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 3,220 | 2,888
Marsh Insurance Group | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 2,325 | 2,061
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 895 | 827
Mercer Consulting Group | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,288 | 1,251
Oliver Wyman Group Consulting Group | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 585 | $ 511

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