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Segment Information (Details For MMC's Operating Segments) (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
Operating income |  | 686 | $ 535 | $ 701 | $ 749 | 580 | $ 517 | $ 661 | $ 673 | 2,655 | 2,431 | 2,184
Total Assets |  | 20,429 |  |  |  | 18,190 |  |  |  | 20,429 | 18,190 | 18,216
Depreciation and Amortization |  |  |  |  |  |  |  |  |  | 481 | 438 | 423
Capital Expenditures |  |  |  |  |  |  |  |  |  | 302 | 253 | 325
Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest income, fiduciary assets |  |  |  |  |  |  |  |  |  | 39 | 26 | 21
Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest income, fiduciary assets |  |  |  |  |  |  |  |  |  | 4 | 3 | 4
Operating Segments |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 14,074 | 13,255 | 12,933
Operating income |  |  |  |  |  |  |  |  |  | 2,841 | 2,619 | 2,380
Total Assets |  | 24,690 |  |  |  | 21,498 |  |  |  | 24,690 | 21,498 | 19,775
Depreciation and Amortization |  |  |  |  |  |  |  |  |  | 411 | 369 | 360
Capital Expenditures |  |  |  |  |  |  |  |  |  | 227 | 196 | 244
Operating Segments | Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue | [1] |  |  |  |  |  |  |  |  | 7,630 | 7,143 | 6,869
Operating income |  |  |  |  |  |  |  |  |  | 1,731 | 1,581 | 1,366
Total Assets |  | 16,490 |  |  |  | 14,728 |  |  |  | 16,490 | 14,728 | 13,290
Depreciation and Amortization |  |  |  |  |  |  |  |  |  | 282 | 248 | 240
Capital Expenditures |  |  |  |  |  |  |  |  |  | 139 | 128 | 136
Interest income, fiduciary assets |  |  |  |  |  |  |  |  |  | 39 | 26 | 21
Equity method income |  |  |  |  |  |  |  |  |  | 14 | 12 | 6
Operating Segments | Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue | [2] |  |  |  |  |  |  |  |  | 6,444 | 6,112 | 6,064
Operating income |  |  |  |  |  |  |  |  |  | 1,110 | 1,038 | 1,014
Total Assets |  | 8,200 |  |  |  | 6,770 |  |  |  | 8,200 | 6,770 | 6,485
Depreciation and Amortization |  |  |  |  |  |  |  |  |  | 129 | 121 | 120
Capital Expenditures |  |  |  |  |  |  |  |  |  | 88 | 68 | 108
Interest income, fiduciary assets |  |  |  |  |  |  |  |  |  | 4 | 3 | 4
Equity method income |  |  |  |  |  |  |  |  |  | 17 | 19 | 21
Corporate and Elimination |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | (50) | (44) | (40)
Operating income |  |  |  |  |  |  |  |  |  | (186) | (188) | (196)
Total Assets | [3] | $ (4,261) |  |  |  | $ (3,308) |  |  |  | (4,261) | (3,308) | (1,559)
Depreciation and Amortization |  |  |  |  |  |  |  |  |  | 70 | 69 | 63
Capital Expenditures |  |  |  |  |  |  |  |  |  | 75 | 57 | 81
Intersegment Eliminations | Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 5 | 6 | 6
Intersegment Eliminations | Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | $ 45 | $ 38 | $ 34
[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.
[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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