Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Segment Reporting Information [Line Items]
Revenue	$ 3,495		$ 3,376		$ 6,998		$ 6,712
Operating Income (Loss)	764		726		1,573		1,459
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,508		3,389		7,023		6,735
Operating Income (Loss)	811		775		1,665		1,555
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1,916	[1]	1,850	[1]	3,905	[2]	3,718	[2]
Operating Income (Loss)	528		490		1,141		1,025
Interest on fiduciary funds (less than)	9		6		17		12
Equity method income	7		6		9		7
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,592	[3]	1,539	[3]	3,118	[4]	3,017	[4]
Operating Income (Loss)	283		285		524		530
Interest on fiduciary funds (less than)	1		1		1		1
Equity method income	5		5		9		9
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	(13)		(13)		(25)		(23)
Operating Income (Loss)	(47)		(49)		(92)		(96)
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	4		3		4		4
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 9		$ 10		$ 21		$ 19

[1] Includes inter-segment revenue of $4 million and $3 million in 2017 and 2016, respectively, interest income on fiduciary funds of $9 million and $6 million in 2017 and 2016, respectively, and equity method income of $7 million and $6 million in 2017 and 2016, respectively.

[2] Includes inter-segment revenue of $4 million in both 2017 and 2016, interest income on fiduciary funds of $17 million and $12 million in 2017 and 2016, respectively, and equity method income of $9 million and $7 million in 2017 and 2016, respectively.

[3] Includes inter-segment revenue of $9 million and $10 million in 2017 and 2016, respectively, interest income on fiduciary funds of less than $1 million in both 2017 and 2016, and equity method income of $5 million in both 2017 and 2016.

[4] Includes inter-segment revenue of $21 million and $19 million in 2017 and 2016, respectively, interest income on fiduciary funds of $1 million in both 2017 and 2016, and equity method income of $9 million in both 2017 and 2016.