Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenue	$ 3,504	$ 3,341	$ 11,238	$ 10,339
Operating Income (Loss)	541	535	2,140	1,985
Equity method income	4		14
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Interest on fiduciary funds (less than)	18		46
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,519	3,350	11,277	10,373
Operating Income (Loss)	584	579	2,286	2,119
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1,863	1,763	6,303	5,668
Operating Income (Loss)	293	268	1,481	1,318
Interest on fiduciary funds (less than)	18	11	46	28
Equity method income	4	2	10	11
Proceeds from divestiture of businesses			51
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,656	1,587	4,974	4,705
Operating Income (Loss)	291	311	805	801
Interest on fiduciary funds (less than)	1	2	3	3
Equity method income	4	3	12	12
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	(15)	(9)	(39)	(34)
Operating Income (Loss)	(43)	(44)	(146)	(134)
Intersegment Eliminations \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	4	1	5	5
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 11	$ 8	$ 34	$ 29

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.

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The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

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