Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenue	$ 4,349	$ 3,734	$ 8,420	$ 7,734
Operating Income (Loss)	680	691	1,618	1,599
Equity method income	5	2	7	9
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Interest on fiduciary funds	26		49
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,374	3,746	8,470	7,758
Operating Income (Loss)	795	739	1,807	1,702
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,574	2,096	4,997	4,440
Operating Income (Loss)	517	472	1,250	1,188
Interest on fiduciary funds	26	15	49	28
Equity method income	4	7	6	6
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,800	1,650	3,473	3,318
Operating Income (Loss)	278	267	557	514
Interest on fiduciary funds	1	1	2	2
Equity method income	5	5	10	8
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	(25)	(12)	(50)	(24)
Operating Income (Loss)	(115)	(48)	(189)	(103)
Intersegment Eliminations \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3		4	1
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 22	$ 12	$ 46	$ 23

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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