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Segment Information (Details of Operating Segment Revenue) (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 Segments |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 3,746 | 3,508 | 7,758 | 7,023
Operating Segments | Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 2,096 | 1,916 | 4,440 | 3,905
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 1,760 | 1,621 | 3,463 | 3,223
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 336 | 295 | 977 | 682
Operating Segments | Consulting Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 1,650 | 1,592 | 3,318 | 3,118
Operating Segments | Consulting Segment | Mercer Consulting Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 1,158 | 1,109 | 2,329 | 2,186
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 492 | 483 | 989 | 932
Corporate and Eliminations |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | $ (12) | $ (13) | $ (24) | $ (25)

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