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Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2022 | Mar. 31, 2021
Segment Reporting Information [Line Items] |  |
Revenue | $ 5,549 | $ 5,083
Operating Income (Loss) | 1,445 | 1,358
Loss on deconsolidation of Russian businesses | (39) | 0
Risk and Insurance Services Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 3,549 | 3,225
Interest on fiduciary funds | 4 | 5
Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,010 | 1,873
Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Revenue | 5,559 | 5,098
Operating Income (Loss) | 1,513 | 1,421
Operating Segments | Risk and Insurance Services Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 3,549 | 3,225
Operating Income (Loss) | 1,121 | 1,060
Interest on fiduciary funds | 4 | 5
Loss on deconsolidation of Russian businesses | (27) |
Operating Segments | Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,010 | 1,873
Operating Income (Loss) | 392 | 361
Corporate/Eliminations |  |
Segment Reporting Information [Line Items] |  |
Revenue | (10) | (15)
Operating Income (Loss) | (68) | (63)
Intersegment Eliminations | Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 10 | $ 15
Loss on deconsolidation of Russian businesses | $ (12) |

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