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Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2020 | Mar. 31, 2019
Segment Reporting Information [Line Items] |  |
Revenue | $ 4,651 | $ 4,071
Risk and Insurance Services Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,911 |
Risk and Insurance Services Segment | Marsh Insurance Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,061 |
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 827 |
Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,762 |
Consulting Segment | Mercer Consulting Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,251 |
Consulting Segment | Oliver Wyman Group Consulting Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 511 |
Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Revenue | 4,673 | 4,096
Operating Segments | Risk and Insurance Services Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,911 | 2,423
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,076 | 1,753
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 835 | 670
Operating Segments | Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,762 | 1,673
Operating Segments | Consulting Segment | Mercer Consulting Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,251 | 1,155
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 511 | 518
Corporate/Eliminations |  |
Segment Reporting Information [Line Items] |  |
Revenue | $ (22) | $ (25)

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