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Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2017 | Mar. 31, 2016
Segment Reporting Information [Line Items] |  |  |
Revenue |  | $ 3,503 | $ 3,336
Operating Income (Loss) |  | 809 | 733
Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 3,515 | 3,346
Operating Income (Loss) |  | 854 | 780
Operating Segments | Risk and Insurance Services Segment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | [1] | 1,989 | 1,868
Operating Income (Loss) |  | 613 | 535
Interest on fiduciary funds (less than) |  | 8 | 6
Equity method income |  | 2 | 1
Operating Segments | Consulting Segment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | [2] | 1,526 | 1,478
Operating Income (Loss) |  | 241 | 245
Interest on fiduciary funds (less than) |  | 1 | 1
Equity method income |  | 4 | 4
Corporate and Eliminations |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | (12) | (10)
Operating Income (Loss) |  | (45) | (47)
Intersegment Eliminations | Risk and Insurance Services Segment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 0 | 1
Intersegment Eliminations | Consulting Segment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | $ 12 | $ 9
[1] | Includes inter-segment revenue of $1 million in 2016, interest income on fiduciary funds of $8 million and $6 million in 2017 and 2016, respectively, and equity method income of $2 million in 2017 and $1 million in 2016, respectively.
[2] | Includes inter-segment revenue of $12 million and $9 million in 2017 and 2016, respectively, interest income on fiduciary funds of $1 million in both 2017 and 2016, and equity method income of $4 million in both 2017 and 2016.

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