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Segment Information (Details For MMC's Operating Segments) (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
Operating income |  | 621 | $ 541 | $ 691 | $ 908 | 670 | $ 535 | $ 701 | $ 749 | 2,761 | 2,655 | 2,431
Total Assets |  | 21,578 |  |  |  | 20,429 |  |  |  | 21,578 | 20,429 | 18,190
Depreciation and Amortization |  |  |  |  |  |  |  |  |  | 494 | 481 | 438
Capital Expenditures |  |  |  |  |  |  |  |  |  | 314 | 302 | 253
Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest income, fiduciary assets |  |  |  |  |  |  |  |  |  | 65 | 39 | 26
Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest income, fiduciary assets |  |  |  |  |  |  |  |  |  | 3 | 4 | 3
Operating Segments |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 15,007 | 14,074 | 13,255
Operating income |  |  |  |  |  |  |  |  |  | 2,963 | 2,841 | 2,619
Total Assets |  | 23,871 |  |  |  | 24,690 |  |  |  | 23,871 | 24,690 | 21,498
Depreciation and Amortization |  |  |  |  |  |  |  |  |  | 420 | 411 | 369
Capital Expenditures |  |  |  |  |  |  |  |  |  | 255 | 227 | 196
Operating Segments | Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue | [1] |  |  |  |  |  |  |  |  | 8,228 | 7,630 | 7,143
Operating income |  |  |  |  |  |  |  |  |  | 1,864 | 1,731 | 1,581
Total Assets |  | 15,868 |  |  |  | 16,490 |  |  |  | 15,868 | 16,490 | 14,728
Depreciation and Amortization |  |  |  |  |  |  |  |  |  | 290 | 282 | 248
Capital Expenditures |  |  |  |  |  |  |  |  |  | 158 | 139 | 128
Interest income, fiduciary assets |  |  |  |  |  |  |  |  |  | 65 | 39 | 26
Equity method income |  |  |  |  |  |  |  |  |  | 13 | 14 | 12
Proceeds from sale of business |  |  |  |  |  |  |  |  |  | 40 |  |
Operating Segments | Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue | [2] |  |  |  |  |  |  |  |  | 6,779 | 6,444 | 6,112
Operating income |  |  |  |  |  |  |  |  |  | 1,099 | 1,110 | 1,038
Total Assets |  | 8,003 |  |  |  | 8,200 |  |  |  | 8,003 | 8,200 | 6,770
Depreciation and Amortization |  |  |  |  |  |  |  |  |  | 130 | 129 | 121
Capital Expenditures |  |  |  |  |  |  |  |  |  | 97 | 88 | 68
Interest income, fiduciary assets |  |  |  |  |  |  |  |  |  | 3 | 4 | 3
Equity method income |  |  |  |  |  |  |  |  |  | 8 | 17 | 19
Corporate and Elimination |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | (57) | (50) | (44)
Operating income |  |  |  |  |  |  |  |  |  | (202) | (186) | (188)
Total Assets | [3] | $ (2,293) |  |  |  | $ (4,261) |  |  |  | (2,293) | (4,261) | (3,308)
Depreciation and Amortization |  |  |  |  |  |  |  |  |  | 74 | 70 | 69
Capital Expenditures |  |  |  |  |  |  |  |  |  | 59 | 75 | 57
Intersegment Eliminations | Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 6 | 5 | 6
Intersegment Eliminations | Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | $ 51 | $ 45 | $ 38
[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.
[3] | Corporate assets primarily include insurance recoverables, pension related assets, the owned portion of the Company headquarters building and intercompany eliminations.

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