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Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2020 | Mar. 31, 2019
Segment Reporting Information [Line Items] |  |
Revenue | $ 4,651 | $ 4,071
Operating Income (Loss) | 1,070 | 938
Risk and Insurance Services Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,911 |
Interest on fiduciary funds | 23 |
Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,762 |
Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Revenue | 4,673 | 4,096
Operating Income (Loss) | 1,136 | 1,012
Operating Segments | Risk and Insurance Services Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,911 | 2,423
Operating Income (Loss) | 854 | 733
Interest on fiduciary funds | 23 | 23
Equity method income | (1) | 2
Operating Segments | Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,762 | 1,673
Operating Income (Loss) | 282 | 279
Interest on fiduciary funds | 1 | 1
Equity method income | 4 | 5
Corporate/Eliminations |  |
Segment Reporting Information [Line Items] |  |
Revenue | (22) | (25)
Operating Income (Loss) | (66) | (74)
Intersegment Eliminations | Risk and Insurance Services Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1 | 1
Intersegment Eliminations | Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | $ 21 | $ 24

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