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Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 5,382 | $ 4,770 | $ 17,182 | $ 15,698
Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 5,396 | 4,789 | 17,227 | 15,745
Risk and Insurance Services | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 3,190 | 2,838 | 10,818 | 9,700
Fiduciary interest income | 131 | 40 | 330 | 57
Loss on consolidation of business |  |  |  | (27)
Consulting | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,206 | 1,951 | 6,409 | 6,045
Marsh & Guy Carpenter | Risk and Insurance Services | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 3,059 | 2,798 | 10,488 | 9,643
Marsh | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Loss on consolidation of business |  |  |  | (27)
Marsh | Risk and Insurance Services | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,700 | 2,470 | 8,482 | 7,794
Marsh | Risk and Insurance Services | Total International | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,137 | 1,019 | 3,848 | 3,531
Marsh | Risk and Insurance Services | EMEA | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 692 | 615 | 2,482 | 2,264
Marsh | Risk and Insurance Services | Asia Pacific | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 311 | 286 | 980 | 927
Marsh | Risk and Insurance Services | Latin America | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 134 | 118 | 386 | 340
Marsh | Risk and Insurance Services | U.S./Canada | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,563 | 1,451 | 4,634 | 4,263
Guy Carpenter | Risk and Insurance Services | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 359 | 328 | 2,006 | 1,849
Mercer | Consulting | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,425 | 1,284 | 4,143 | 4,016
Mercer | Consulting | Wealth | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 635 | 561 | 1,853 | 1,775
Mercer | Consulting | Health | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 496 | 451 | 1,559 | 1,562
Mercer | Consulting | Career | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 294 | 272 | 731 | 679
Oliver Wyman Group | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Loss on consolidation of business |  |  |  | (12)
Oliver Wyman Group | Consulting | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 781 | $ 667 | 2,266 | 2,029
Small Individual Financial Advisory Business | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Pre-tax gain (loss) on sale |  |  | $ (17) |
Mercer U.S. Affinity Business | Operating Segments |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Pre-tax gain (loss) on sale |  |  |  | $ 112

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