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Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | $ 3,341 |  | $ 3,135 |  | $ 10,339 |  | $ 9,847 |
Operating Segments |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 3,350 |  | 3,145 |  | 10,373 |  | 9,880 |
Operating Segments | Risk and Insurance Services Segment |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 1,763 | [1] | 1,632 | [1] | 5,668 | [2] | 5,350 | [2]
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 1,490 |  | 1,370 |  | 4,713 |  | 4,427 |
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 273 |  | 262 |  | 955 |  | 923 |
Operating Segments | Consulting Segment |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 1,587 | [3] | 1,513 | [3] | 4,705 | [4] | 4,530 | [4]
Operating Segments | Consulting Segment | Mercer Consulting Group |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 1,149 |  | 1,109 |  | 3,335 |  | 3,227 |
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 438 |  | 404 |  | 1,370 |  | 1,303 |
Corporate and Eliminations |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | $ (9) |  | $ (10) |  | $ (34) |  | $ (33) |
[1] | Includes inter-segment revenue of $1 million in both 2017 and 2016, interest income on fiduciary funds of $11 million and $8 million in 2017 and 2016, respectively, and equity method income of $2 million and $4 million in 2017 and 2016, respectively.
[2] | Includes inter-segment revenue of $5 million in both 2017 and 2016, interest income on fiduciary funds of $28 million and $20 million in 2017 and 2016, respectively, and equity method income of $11 million in both 2017 and 2016.
[3] | Includes inter-segment revenue of $8 million and $9 million in 2017 and 2016, respectively, interest income on fiduciary funds of $2 million in 2017 and $1 million in 2016, respectively, and equity method income of $3 million in 2017 and $5 million in 2016, respectively.
[4] | Includes inter-segment revenue of $29 million and $28 million in 2017 and 2016, respectively, interest income on fiduciary funds of $3 million in 2017 and $2 million in 2016, respectively, and equity method income of $12 million in 2017 and $14 million in 2016, respectively.

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