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Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Segment Reporting Information [Line Items] |  |
Revenue | $ 4,000 | $ 3,503
Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Revenue | 4,012 | 3,515
Operating Segments | Risk and Insurance Services Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,344 | 1,989
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,703 | 1,602
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 641 | 387
Operating Segments | Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,668 | 1,526
Operating Segments | Consulting Segment | Mercer Consulting Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,171 | 1,077
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 497 | 449
Corporate and Eliminations |  |
Segment Reporting Information [Line Items] |  |
Revenue | $ (12) | $ (12)

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