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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Disaggregation of Revenue [Line Items] |  |  |
Revenue | $ 19,820 | $ 17,224 | $ 16,652
Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 19,874 | 17,313 | 16,742
Asia Pacific |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,617 | 2,093 | 2,001
Risk and insurance services |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 12,085 | 10,337 | 9,599
Fiduciary interest income | 15 | 46 | 105
Risk and insurance services | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 12,085 | 10,337 | 9,599
Fiduciary interest income | 15 | 46 | 105
Risk and insurance services | Total International |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 4,861 | 4,058 | 3,895
Risk and insurance services | EMEA |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,946 | 2,575 | 2,482
Risk and insurance services | Asia Pacific |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 1,462 | 1,059 | 953
Risk and insurance services | Latin America |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 453 | 424 | 460
Risk and insurance services | U.S./Canada |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 5,342 | 4,537 | 4,119
Consulting |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 7,789 | 6,976 | 7,143
Consulting | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 7,789 | 6,976 | 7,143
Consulting | Wealth |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,509 | 2,348 | 2,369
Consulting | Health |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 1,855 | 1,793 | 1,796
Consulting | Career |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 890 | 787 | 856
Marsh & Guy Carpenter | Risk and insurance services |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 12,070 | 10,291 | 9,494
Marsh | Risk and insurance services |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 10,203 | 8,595 | 8,014
Marsh | Risk and insurance services | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 10,214 | 8,628 | 8,085
Guy Carpenter | Risk and insurance services |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 1,867 | 1,696 | 1,480
Guy Carpenter | Risk and insurance services | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 1,871 | 1,709 | 1,514
Mercer | Consulting |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 5,254 | 4,928 | 5,021
Mercer | Consulting | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 5,254 | 4,928 | 5,021
Oliver Wyman Group | Consulting |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | 2,535 | 2,048 | 2,122
Oliver Wyman Group | Consulting | Operating Segments |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue | $ 2,535 | $ 2,048 | $ 2,122

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