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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, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  | $ 3,685 | $ 3,341 | $ 3,495 | $ 3,503 | $ 3,364 | $ 3,135 | $ 3,376 | $ 3,336 | $ 14,024 | $ 13,211 | $ 12,893
Fixed assets, net |  | 712 |  |  |  | 725 |  |  |  | 712 | 725 | 773
United States |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 6,870 | 6,573 | 6,316
Fixed assets, net |  | 399 |  |  |  | 412 |  |  |  | 399 | 412 | 460
United Kingdom |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 2,112 | 2,019 | 2,036
Fixed assets, net |  | 91 |  |  |  | 94 |  |  |  | 91 | 94 | 115
Europe |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 2,197 | 2,022 | 1,902
Fixed assets, net |  | 57 |  |  |  | 53 |  |  |  | 57 | 53 | 57
Asia Pacific |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 1,517 | 1,363 | 1,333
Fixed assets, net |  | 78 |  |  |  | 76 |  |  |  | 78 | 76 | 49
Other Geographic Areas |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 1,378 | 1,278 | 1,346
Fixed assets, net |  | $ 87 |  |  |  | $ 90 |  |  |  | 87 | 90 | 92
Operating Segments |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 14,074 | 13,255 | 12,933
Operating Segments | Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue | [1] |  |  |  |  |  |  |  |  | 7,630 | 7,143 | 6,869
Operating Segments | Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue | [2] |  |  |  |  |  |  |  |  | 6,444 | 6,112 | 6,064
Corporate and Elimination |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | (50) | (44) | (40)
Marsh Insurance Group | Operating Segments | Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 6,433 | 5,997 | 5,745
Guy Carpenter Reinsurance Group | Operating Segments | Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 1,197 | 1,146 | 1,124
Mercer Consulting Group | Operating Segments | Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 4,528 | 4,323 | 4,313
Oliver Wyman Group Consulting Group | Operating Segments | Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | $ 1,916 | $ 1,789 | $ 1,751
[1] | Includes inter-segment revenue of $5 million in 2017 and $6 million in both 2016 and 2015, interest income on fiduciary funds of $39 million, $26 million and $21 million in 2017, 2016 and 2015, respectively, and equity method income of $14 million, $12 million and $6 million in 2017, 2016 and 2015, respectively.
[2] | Includes inter-segment revenue of $45 million, $38 million and $34 million in 2017, 2016 and 2015, respectively, interest income on fiduciary funds of $4 million, $3 million and $4 million in 2017, 2016 and 2015, respectively and equity method income of $17 million, $19 million and $21 million in 2017, 2016 and 2015, respectively.

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