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Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2023 | Mar. 31, 2022
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 5,924 | $ 5,549
Operating Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 5,937 | 5,559
Risk and Insurance Services |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 3,906 | 3,549
Fiduciary interest income | 91 | 4
Risk and Insurance Services | Operating Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 3,906 | 3,549
Fiduciary interest income | 91 | 4
Gain (loss) on consolidation of business | (27) |
Risk and Insurance Services | Total International |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,359 | 1,267
Risk and Insurance Services | EMEA |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 932 | 869
Risk and Insurance Services | Asia Pacific |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 312 | 294
Risk and Insurance Services | Latin America |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 115 | 104
Risk and Insurance Services | U.S./Canada |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,385 | 1,279
Consulting |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 2,031 | 2,010
Consulting | Operating Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 2,031 | 2,010
Consulting | Wealth |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 581 | 617
Consulting | Health |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 545 | 524
Consulting | Career |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 218 | 202
Oliver Wyman Group | Operating Segments |  |
Disaggregation of Revenue [Line Items] |  |
Gain (loss) on consolidation of business | (12) |
Small Individual Financial Advisory Business | Operating Segments |  |
Disaggregation of Revenue [Line Items] |  |
Pre-tax loss on sale | 19 |
Marsh & Guy Carpenter | Risk and Insurance Services |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 3,815 | 3,545
Marsh | Risk and Insurance Services |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 2,744 | 2,546
Marsh | Risk and Insurance Services | Operating Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 2,800 | 2,549
Guy Carpenter | Risk and Insurance Services |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,071 | 999
Guy Carpenter | Risk and Insurance Services | Operating Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,106 | 1,000
Mercer | Consulting |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,344 | 1,343
Mercer | Consulting | Operating Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,344 | 1,343
Oliver Wyman Group | Consulting |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 687 | 667
Oliver Wyman Group | Consulting | Operating Segments |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 687 | $ 667

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