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Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2022 | Jun. 30, 2021 | Jun. 30, 2022 | Jun. 30, 2021
Segment Reporting Information [Line Items] |  |  |  |
Revenue | $ 5,379 | $ 5,017 | $ 10,928 | $ 10,100
Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 3,313 | 3,141 | 6,862 | 6,366
Risk and Insurance Services Segment | Marsh Insurance Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 2,778 | 2,650 | 5,324 | 4,975
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 522 | 488 | 1,521 | 1,383
Consulting Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 2,084 | 1,892 | 4,094 | 3,765
Consulting Segment | Mercer Consulting Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 1,389 | 1,274 | 2,732 | 2,562
Consulting Segment | Oliver Wyman Group Consulting Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 695 | 618 | 1,362 | 1,203
Operating Segments |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 5,397 | 5,033 | 10,956 | 10,131
Operating Segments | Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 3,313 | 3,141 | 6,862 | 6,366
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 2,787 | 2,652 | 5,336 | 4,981
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 526 | 489 | 1,526 | 1,385
Operating Segments | Consulting Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 2,084 | 1,892 | 4,094 | 3,765
Operating Segments | Consulting Segment | Mercer Consulting Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 1,389 | 1,274 | 2,732 | 2,562
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | 695 | 618 | 1,362 | 1,203
Corporate/Eliminations |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenue | $ (18) | $ (16) | $ (28) | $ (31)

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