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Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | $ 3,495 |  | $ 3,376 |  | $ 6,998 |  | $ 6,712 |
Operating Segments |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 3,508 |  | 3,389 |  | 7,023 |  | 6,735 |
Operating Segments | Risk and Insurance Services Segment |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 1,916 | [1] | 1,850 | [1] | 3,905 | [2] | 3,718 | [2]
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 1,621 |  | 1,564 |  | 3,223 |  | 3,057 |
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 295 |  | 286 |  | 682 |  | 661 |
Operating Segments | Consulting Segment |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 1,592 | [3] | 1,539 | [3] | 3,118 | [4] | 3,017 | [4]
Operating Segments | Consulting Segment | Mercer Consulting Group |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 1,109 |  | 1,079 |  | 2,186 |  | 2,118 |
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | 483 |  | 460 |  | 932 |  | 899 |
Corporate and Eliminations |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |
Revenue | $ (13) |  | $ (13) |  | $ (25) |  | $ (23) |
[1] | Includes inter-segment revenue of $4 million and $3 million in 2017 and 2016, respectively, interest income on fiduciary funds of $9 million and $6 million in 2017 and 2016, respectively, and equity method income of $7 million and $6 million in 2017 and 2016, respectively.
[2] | Includes inter-segment revenue of $4 million in both 2017 and 2016, interest income on fiduciary funds of $17 million and $12 million in 2017 and 2016, respectively, and equity method income of $9 million and $7 million in 2017 and 2016, respectively.
[3] | Includes inter-segment revenue of $9 million and $10 million in 2017 and 2016, respectively, interest income on fiduciary funds of less than $1 million in both 2017 and 2016, and equity method income of $5 million in both 2017 and 2016.
[4] | Includes inter-segment revenue of $21 million and $19 million in 2017 and 2016, respectively, interest income on fiduciary funds of $1 million in both 2017 and 2016, and equity method income of $9 million in both 2017 and 2016.

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