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v3.21.1

Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Segment Reporting Information [Line Items] |  |
Revenue | $ 5,083 | $ 4,651
Risk and Insurance Services Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 3,225 | 2,911
Risk and Insurance Services Segment | Marsh Insurance Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,325 | 2,061
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 895 | 827
Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,873 | 1,762
Consulting Segment | Mercer Consulting Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,288 | 1,251
Consulting Segment | Oliver Wyman Group Consulting Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 585 | 511
Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Revenue | 5,098 | 4,673
Operating Segments | Risk and Insurance Services Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 3,225 | 2,911
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 2,329 | 2,076
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 896 | 835
Operating Segments | Consulting Segment |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,873 | 1,762
Operating Segments | Consulting Segment | Mercer Consulting Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 1,288 | 1,251
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group |  |
Segment Reporting Information [Line Items] |  |
Revenue | 585 | 511
Corporate/Eliminations |  |
Segment Reporting Information [Line Items] |  |
Revenue | $ (15) | $ (22)

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