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Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Segment Reporting Information [Line Items] |  |  |  |
Revenue | $ 3,504 | $ 3,341 | $ 11,238 | $ 10,339
Operating Income (Loss) | 541 | 535 | 2,140 | 1,985
Equity method income | 4 |  | 14 |
Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Interest on fiduciary funds (less than) | 18 |  | 46 |
Operating Segments |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 3,519 | 3,350 | 11,277 | 10,373
Operating Income (Loss) | 584 | 579 | 2,286 | 2,119
Operating Segments | Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 1,863 | 1,763 | 6,303 | 5,668
Operating Income (Loss) | 293 | 268 | 1,481 | 1,318
Interest on fiduciary funds (less than) | 18 | 11 | 46 | 28
Equity method income | 4 | 2 | 10 | 11
Proceeds from divestiture of businesses |  |  | 51 |
Operating Segments | Consulting Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 1,656 | 1,587 | 4,974 | 4,705
Operating Income (Loss) | 291 | 311 | 805 | 801
Interest on fiduciary funds (less than) | 1 | 2 | 3 | 3
Equity method income | 4 | 3 | 12 | 12
Corporate and Eliminations |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | (15) | (9) | (39) | (34)
Operating Income (Loss) | (43) | (44) | (146) | (134)
Intersegment Eliminations | Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 4 | 1 | 5 | 5
Intersegment Eliminations | Consulting Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | $ 11 | $ 8 | $ 34 | $ 29

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