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Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2017 | Mar. 31, 2016
Segment Reporting Information [Line Items] |  |  |
Revenue |  | $ 3,503 | $ 3,336
Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 3,515 | 3,346
Operating Segments | Risk and Insurance Services Segment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | [1] | 1,989 | 1,868
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 1,602 | 1,493
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 387 | 375
Operating Segments | Consulting Segment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | [2] | 1,526 | 1,478
Operating Segments | Consulting Segment | Mercer Consulting Group |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 1,077 | 1,039
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 449 | 439
Corporate and Eliminations |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | $ (12) | $ (10)
[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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