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Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
Segment Reporting Information [Line Items] |  |  |
Revenue |  | $ 4,071 | $ 4,000
Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 4,096 | 4,012
Operating Segments | Risk and Insurance Services Segment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | [1] | 2,423 | 2,344
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 1,753 | 1,703
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 670 | 641
Operating Segments | Consulting Segment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue | [2] | 1,673 | 1,668
Operating Segments | Consulting Segment | Mercer Consulting Group |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 1,155 | 1,171
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | 518 | 497
Corporate and Eliminations |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenue |  | $ (25) | $ (12)
[1] | Includes inter-segment revenue of $1 million in 2019 and 2018, respectively, interest income on fiduciary funds of $23 million and $13 million in 2019 and 2018, respectively, and equity method income (loss) of $2 million and $(1) million in 2019 and 2018, respectively
[2] | Includes inter-segment revenue of $24 million and $11 million in 2019 and 2018, respectively, interest income on fiduciary funds of $1 million in 2019 and 2018, respectively, and equity method income of $5 million and $3 million in 2019 and in 2018, respectively.

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