Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenue	$ 3,968	$ 3,504	$ 12,388	$ 11,238
Operating Income (Loss)	467	541	2,085	2,140
Equity method income	3	4	10	14
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Interest on fiduciary funds	31		80
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,991	3,519	12,461	11,277
Operating Income (Loss)	535	584	2,342	2,286
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,206	1,863	7,203	6,303
Operating Income (Loss)	218	293	1,468	1,481
Interest on fiduciary funds	31	18	80	46
Equity method income	10	4	16	10
Proceeds from divestiture of businesses		51		51
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,785	1,656	5,258	4,974
Operating Income (Loss)	317	291	874	805
Interest on fiduciary funds	1	1	2	3
Equity method income		4	10	12
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(23)	(15)	(73)	(39)
Operating Income (Loss)	(68)	(43)	(257)	(146)
Intersegment Eliminations \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3	4	7	5
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 20	$ 11	$ 66	$ 34

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

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- Definition

Amount of income (loss) for proportionate share of equity method investee's income (loss).

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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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Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.

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