Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting Information [Line Items]
Revenue	$ 3,300		$ 3,088		$ 6,564		$ 6,214
Operating Income (Loss)	647		577		1,320		1,184
Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Interest on fiduciary funds					12		14
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Interest on fiduciary funds	2		1		3		2
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	3,311		3,098		6,582		6,231
Operating Income (Loss)	695		626		1,413		1,281
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,791	[1]	1,688	[1]	3,630	[2]	3,459	[2]
Operating Income (Loss)	448		421		941		889
Interest on fiduciary funds	6		6		12		14
Equity method income	6		7		6		8
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,520	[3]	1,410	[3]	2,952	[4]	2,772	[4]
Operating Income (Loss)	247		205		472		392
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(11)		(10)		(18)		(17)
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	3		3		3		3
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	8		7		15		14
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ (48)		$ (49)		$ (93)		$ (97)

[1] Includes inter-segment revenue of $3 million in both 2014 and 2013, interest income on fiduciary funds of $6 million in both 2014 and 2013, and equity method income of $6 million and $7 million in 2014 and 2013, respectively.

[2] Includes inter-segment revenue of $3 million in both 2014 and 2013, interest income on fiduciary funds of $12 million and $14 million in 2014 and 2013, respectively, and equity method income of $6 million and $8 million in 2014 and 2013, respectively.

[3] Includes inter-segment revenue of $8 million and $7 million in 2014 and 2013, respectively, and interest income on fiduciary funds of $2 million in 2014 and $1 million in 2013.

[4] Includes inter-segment revenue of $15 million and $14 million in 2014 and 2013, respectively, and interest income on fiduciary funds of $3 million and $2 million in 2014 and 2013, respectively.