Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenue	$ 4,000	$ 3,503
Operating Income (Loss)	908	749
Equity method income	7
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Interest on fiduciary funds (less than)	13
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,012	3,515
Operating Income (Loss)	963	793
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,344	1,989
Operating Income (Loss)	716	568
Interest on fiduciary funds (less than)	13	8
Equity method income	(1)	2
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,668	1,526
Operating Income (Loss)	247	225
Interest on fiduciary funds (less than)	1	1
Equity method income	3	4
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	(12)	(12)
Operating Income (Loss)	(55)	(44)
Intersegment Eliminations \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 11	$ 12

Interest income earned on both U.S. and foreign fiduciary funds held in trust.

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- Definition

This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(12))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(10))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(f))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

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- Definition

Aggregate revenue during the period from services rendered in the normal course of business, after deducting allowances and discounts.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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