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Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Segment Reporting Information [Line Items] |  |  |  |
Revenue | $ 3,734 | $ 3,495 | $ 7,734 | $ 6,998
Operating Income (Loss) | 691 | 701 | 1,599 | 1,450
Equity method income | 2 |  | 9 |
Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Interest on fiduciary funds (less than) | 15 |  | 28 |
Operating Segments |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 3,746 | 3,508 | 7,758 | 7,023
Operating Income (Loss) | 739 | 747 | 1,702 | 1,540
Operating Segments | Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 2,096 | 1,916 | 4,440 | 3,905
Operating Income (Loss) | 472 | 482 | 1,188 | 1,050
Interest on fiduciary funds (less than) | 15 | 9 | 28 | 17
Equity method income | 7 | 7 | 6 | 9
Operating Segments | Consulting Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 1,650 | 1,592 | 3,318 | 3,118
Operating Income (Loss) | 267 | 265 | 514 | 490
Interest on fiduciary funds (less than) | 1 | 1 | 2 | 1
Equity method income | 5 | 5 | 8 | 9
Corporate and Eliminations |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | (12) | (13) | (24) | (25)
Operating Income (Loss) | (48) | (46) | (103) | (90)
Intersegment Eliminations | Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue |  | 4 | 1 | 4
Intersegment Eliminations | Consulting Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | $ 12 | $ 9 | $ 23 | $ 21

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