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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2024 | Mar. 31, 2023
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 6,473 | $ 5,924
Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 6,487 | 5,937
Risk and Insurance Services | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Fiduciary interest income | 122 | 91
Revenue | 4,273 | 3,906
Risk and Insurance Services | Subtotal | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, excluding fiduciary interest income | 4,151 | 3,815
Risk and Insurance Services | Total Marsh | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, excluding fiduciary interest income | 3,003 | 2,744
Risk and Insurance Services | Guy Carpenter | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, excluding fiduciary interest income | 1,148 | 1,071
Risk and Insurance Services | Total International | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, excluding fiduciary interest income | 1,486 | 1,359
Risk and Insurance Services | EMEA | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, excluding fiduciary interest income | 1,025 | 932
Risk and Insurance Services | Asia Pacific | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, excluding fiduciary interest income | 336 | 312
Risk and Insurance Services | Latin America | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, excluding fiduciary interest income | 125 | 115
Risk and Insurance Services | U.S./Canada | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, excluding fiduciary interest income | 1,517 | 1,385
Consulting | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 2,214 | 2,031
Consulting | Mercer | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,425 | 1,344
Consulting | Mercer | Wealth | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 672 | 581
Consulting | Mercer | Health | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 538 | 545
Consulting | Mercer | Career | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 215 | 218
Consulting | Oliver Wyman Group | Total Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 789 | $ 687

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