Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2016		Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Segment Reporting Information [Line Items]
Revenue	$ 3,376		$ 3,225		$ 6,712		$ 6,440
Operating Income (Loss)	726		629		1,459		1,364
Interest on fiduciary funds (less than)	6		5		12		10
Consulting Segment
Segment Reporting Information [Line Items]
Interest on fiduciary funds (less than)	1		1		1		2
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,389		3,237		6,735		6,461
Operating Income (Loss)	775		675		1,555		1,456
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1,850	[1]	1,750	[1]	3,718	[2]	3,553	[2]
Operating Income (Loss)	490		427		1,025		960
Interest on fiduciary funds (less than)	6		5		12	[2]	10	[2]
Equity method income	6		0		7	[2]	2	[2]
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,539	[3]	1,487	[3]	3,017	[4]	2,908	[4]
Operating Income (Loss)	285		248		530		496
Interest on fiduciary funds (less than)	1		1		1		2
Equity method income	5		5		9	[4]	8
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	(13)		(12)		(23)		(21)
Operating Income (Loss)	$ (49)		$ (46)		$ (96)		$ (92)

[1] Includes inter-segment revenue of $3 million and $4 million in 2016 and 2015, respectively, interest income on fiduciary funds of $6 million and $5 million in 2016 and 2015, respectively, and equity method income of $6 million in 2016 and $0 million in 2015, respectively.

[2] Includes inter-segment revenue of $4 million in 2016 and $5 million in 2015, interest income on fiduciary funds of $12 million and $10 million in 2016 and 2015, respectively, and equity method income of $7 million and $2 million in 2016 and 2015, respectively.

[3] Includes inter-segment revenue of $10 million and $8 million in 2016 and 2015, respectively, interest income on fiduciary funds of less than $1 million and $1 million in 2016 and 2015, respectively, and equity method income of $5 million in both 2016 and 2015.

[4] Includes inter-segment revenue of $19 million and $16 million in 2016 and 2015, respectively, interest income on fiduciary funds of $1 million and $2 million in 2016 and 2015, respectively, and equity method income of $9 million and $8 million in 2016 and 2015, respectively.