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Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Segment Reporting Information [Line Items] |  |  |
Revenue | $ 20,720 | $ 19,820 | $ 17,224
Fixed Assets, Net | 871 | 847 | 856
United States |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 10,215 | 9,343 | 8,168
Fixed Assets, Net | 473 | 484 | 492
United Kingdom |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 3,114 | 3,130 | 2,818
Fixed Assets, Net | 166 | 116 | 115
Continental Europe |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 3,223 | 3,219 | 2,881
Fixed Assets, Net | 68 | 68 | 74
Asia Pacific |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 2,537 | 2,617 | 2,093
Fixed Assets, Net | 81 | 96 | 105
Other |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 1,695 | 1,565 | 1,353
Fixed Assets, Net | 83 | 83 | 70
Risk and Insurance Services |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 12,645 | 12,085 | 10,337
Risk and Insurance Services | Asia Pacific |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 1,333 | 1,462 | 1,059
Consulting |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 8,139 | 7,789 | 6,976
Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 20,784 | 19,874 | 17,313
Pre-tax gain on sale |  | (50) |
Operating Segments | Risk and Insurance Services |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 12,645 | 12,085 | 10,337
Pre-tax gain on sale |  | (50) |
Gain (loss) on consolidation of business | 27 |  |
Operating Segments | Consulting |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 8,139 | 7,789 | 6,976
Operating Segments | Marsh India |  |  |
Segment Reporting Information [Line Items] |  |  |
Pre-tax gain on sale |  | (267) |
Operating Segments | Mercer U.S. Affinity Business |  |  |
Segment Reporting Information [Line Items] |  |  |
Pre-tax gain on sale | (112) |  |
Operating Segments | Oliver Wyman Group |  |  |
Segment Reporting Information [Line Items] |  |  |
Gain (loss) on consolidation of business | 12 |  |
Corporate/Eliminations |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | (64) | (54) | (89)
Marsh | Risk and Insurance Services |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 10,505 | 10,203 | 8,595
Marsh | Operating Segments | Risk and Insurance Services |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 10,585 | 10,214 | 8,628
Guy Carpenter | Risk and Insurance Services |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 2,020 | 1,867 | 1,696
Guy Carpenter | Operating Segments | Risk and Insurance Services |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 2,060 | 1,871 | 1,709
Mercer | Consulting |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 5,345 | 5,254 | 4,928
Mercer | Operating Segments | Consulting |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 5,345 | 5,254 | 4,928
Oliver Wyman Group | Consulting |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | 2,794 | 2,535 | 2,048
Oliver Wyman Group | Operating Segments | Consulting |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | $ 2,794 | $ 2,535 | $ 2,048

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