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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Disaggregation of Revenue [Line Items] |  |  |
Revenue | $ 20,720 | $ 19,820 | $ 17,224
Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 20,784 | 19,874 | 17,313
Pre-tax gain on sale |  | 50 |
Asia Pacific |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,537 | 2,617 | 2,093
Risk and Insurance Services |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 12,645 | 12,085 | 10,337
Fiduciary interest income | 120 | 15 | 46
Risk and Insurance Services | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 12,645 | 12,085 | 10,337
Fiduciary interest income | 120 | 15 | 46
Gain (loss) on consolidation of business | (27) |  |
Pre-tax gain on sale |  | 50 |
Risk and Insurance Services | Marsh & Guy Carpenter |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 12,525 | 12,070 | 10,291
Risk and Insurance Services | Marsh |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 10,505 | 10,203 | 8,595
Risk and Insurance Services | Marsh | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 10,585 | 10,214 | 8,628
Risk and Insurance Services | Guy Carpenter |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,020 | 1,867 | 1,696
Risk and Insurance Services | Guy Carpenter | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,060 | 1,871 | 1,709
Risk and Insurance Services | Total International |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 4,714 | 4,861 | 4,058
Risk and Insurance Services | EMEA |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,879 | 2,946 | 2,575
Risk and Insurance Services | Asia Pacific |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 1,333 | 1,462 | 1,059
Risk and Insurance Services | Latin America |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 502 | 453 | 424
Risk and Insurance Services | U.S./Canada |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 5,791 | 5,342 | 4,537
Consulting |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 8,139 | 7,789 | 6,976
Consulting | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 8,139 | 7,789 | 6,976
Consulting | Wealth |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,366 | 2,509 | 2,348
Consulting | Health |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,017 | 1,855 | 1,793
Consulting | Career |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 962 | 890 | 787
Consulting | Mercer |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 5,345 | 5,254 | 4,928
Consulting | Mercer | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 5,345 | 5,254 | 4,928
Consulting | Oliver Wyman Group |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,794 | 2,535 | 2,048
Consulting | Oliver Wyman Group | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,794 | 2,535 | $ 2,048
Oliver Wyman Group | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Gain (loss) on consolidation of business | (12) |  |
Marsh India | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Pre-tax gain on sale |  | $ 267 |
Mercer U.S. Affinity Business | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Pre-tax gain on sale | $ 112 |  |

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