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Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  | $ 3,712 | $ 3,504 | $ 3,734 | $ 4,000 | $ 3,685 | $ 3,341 | $ 3,495 | $ 3,503 | $ 14,950 | $ 14,024 | $ 13,211
Fixed assets, net |  | 701 |  |  |  | 712 |  |  |  | 701 | 712 | 725
United States |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 7,219 | 6,870 | 6,573
Fixed assets, net |  | 403 |  |  |  | 399 |  |  |  | 403 | 399 | 412
United Kingdom |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 2,243 | 2,112 | 2,019
Fixed assets, net |  | 91 |  |  |  | 91 |  |  |  | 91 | 91 | 94
Europe |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 2,694 | 2,197 | 2,022
Fixed assets, net |  | 59 |  |  |  | 57 |  |  |  | 59 | 57 | 53
Asia Pacific |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 1,616 | 1,517 | 1,363
Fixed assets, net |  | 74 |  |  |  | 78 |  |  |  | 74 | 78 | 76
Other Geographic Areas |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 1,235 | 1,378 | 1,278
Fixed assets, net |  | $ 74 |  |  |  | $ 87 |  |  |  | 74 | 87 | 90
Operating Segments |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 15,007 | 14,074 | 13,255
Operating Segments | Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue | [1] |  |  |  |  |  |  |  |  | 8,228 | 7,630 | 7,143
Operating Segments | Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue | [2] |  |  |  |  |  |  |  |  | 6,779 | 6,444 | 6,112
Corporate and Elimination |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | (57) | (50) | (44)
Marsh Insurance Group | Operating Segments | Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 6,923 | 6,433 | 5,997
Guy Carpenter Reinsurance Group | Operating Segments | Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 1,305 | 1,197 | 1,146
Mercer Consulting Group | Operating Segments | Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 4,732 | 4,528 | 4,323
Oliver Wyman Group Consulting Group | Operating Segments | Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | $ 2,047 | $ 1,916 | $ 1,789
[1] | Includes inter-segment revenue of $6 million, $5 million and $6 million in 2018, 2017 and 2016, respectively, interest income on fiduciary funds of $65 million, $39 million and $26 million in 2018, 2017 and 2016, respectively, and equity method income of $13 million, $14 million and $12 million in 2018, 2017 and 2016, respectively and $40 million related to the sale of business in 2018.
[2] | Includes inter-segment revenue of $51 million, $45 million and $38 million in 2018, 2017 and 2016, respectively, interest income on fiduciary funds of $3 million, $4 million and $3 million in 2018, 2017 and 2016, respectively, and equity method income of $8 million, $17 million and $19 million in 2018, 2017 and 2016, respectively.

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