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Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2024 | Mar. 31, 2023
Segment Reporting Information [Line Items] |  |
Revenue | $ 6,473 | $ 5,924
Operating Income/(Loss) | 1,925 | 1,726
Total Segments |  |
Segment Reporting Information [Line Items] |  |
Revenue | 6,487 | 5,937
Operating Income/(Loss) | 1,997 | 1,806
Total Segments | Discontinued Operations, Disposed of by Sale | Small Individual Financial Advisory Business |  |
Segment Reporting Information [Line Items] |  |
Net gain includes in revenue consolidated statements of income | 21 |
Total Segments | Small Individual Financial Advisory Business |  |
Segment Reporting Information [Line Items] |  |
Net gain includes in revenue consolidated statements of income |  | (19)
Total Segments | Risk and Insurance Services |  |
Segment Reporting Information [Line Items] |  |
Revenue | 4,273 | 3,906
Operating Income/(Loss) | 1,565 | 1,395
Interest on fiduciary funds | 122 | 91
Total Segments | Consulting |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,214 | 2,031
Operating Income/(Loss) | 432 | 411
Total Segments | Consulting | Oliver Wyman Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 789 | 687
Corporate/Eliminations |  |
Segment Reporting Information [Line Items] |  |
Revenue | (14) | (13)
Operating Income/(Loss) | (72) | (80)
Intersegment Eliminations | Consulting |  |
Segment Reporting Information [Line Items] |  |
Revenue | $ 13 | $ 13

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