Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenue	$ 3,734	$ 3,495	$ 7,734	$ 6,998
Operating Income (Loss)	691	701	1,599	1,450
Equity method income	2		9
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Interest on fiduciary funds (less than)	15		28
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,746	3,508	7,758	7,023
Operating Income (Loss)	739	747	1,702	1,540
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,096	1,916	4,440	3,905
Operating Income (Loss)	472	482	1,188	1,050
Interest on fiduciary funds (less than)	15	9	28	17
Equity method income	7	7	6	9
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,650	1,592	3,318	3,118
Operating Income (Loss)	267	265	514	490
Interest on fiduciary funds (less than)	1	1	2	1
Equity method income	5	5	8	9
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	(12)	(13)	(24)	(25)
Operating Income (Loss)	(48)	(46)	(103)	(90)
Intersegment Eliminations \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue		4	1	4
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 12	$ 9	$ 23	$ 21

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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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.

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