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, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Revenue	$ 3,088		$ 3,026		$ 6,214		$ 6,077
Operating Income (Loss)	577		518		1,184		1,045
Interest income on fiduciary funds	4		6		8		12
Equity method income			4		20		24
Sales Revenue, Services, Net, Originally Reported			3,026				6,077
Sales Revenue, Services, Net, Adjusted			0				0
Operating Income (Loss), Originally Reported			518				1,045
Operating Income (Loss), Adjusted			0				0
Marsh [Member]
Revenue	1,402		1,364		2,797		2,694
Sales Revenue, Services, Net, Originally Reported			1,421				2,809
Sales Revenue, Services, Net, Adjusted			(57)				(115)
Risk and Insurance Services [Member]
Revenue	1,688	[1]	1,641	[1]	3,459	[2]	3,330	[2]
Operating Income (Loss)	421		390		889		802
Inter-segment revenue	3		2		3		3
Interest income on fiduciary funds	6		10		14		21
Equity method income	7		9		8		10
Sales Revenue, Services, Net, Originally Reported			1,698				3,445
Sales Revenue, Services, Net, Adjusted			(57)				(115)
Operating Income (Loss), Originally Reported			401				818
Operating Income (Loss), Adjusted			(11)				(16)
Consulting [Member]
Revenue	1,410	[3]	1,398	[3]	2,772	[4]	2,769	[4]
Operating Income (Loss)	205		183		392		347
Inter-segment revenue	7		11		14		19
Interest income on fiduciary funds	1		1		2		1
Sales Revenue, Services, Net, Originally Reported			1,341				2,654
Sales Revenue, Services, Net, Adjusted			57				115
Operating Income (Loss), Originally Reported			172				331
Operating Income (Loss), Adjusted			11				16
Total Operating Segments [Member]
Revenue	3,098		3,039		6,231		6,099
Operating Income (Loss)	626		573		1,281		1,149
Sales Revenue, Services, Net, Originally Reported			3,039				6,099
Sales Revenue, Services, Net, Adjusted			0				0
Operating Income (Loss), Originally Reported			573				1,149
Operating Income (Loss), Adjusted			0				0
Corporate / Eliminations [Member]
Revenue	(10)		(13)		(17)		(22)
Operating Income (Loss)	(49)		(55)		(97)		(104)
Sales Revenue, Services, Net, Originally Reported			(13)				(22)
Sales Revenue, Services, Net, Adjusted			0				0
Operating Income (Loss), Originally Reported			(55)				(104)
Operating Income (Loss), Adjusted			0				0
Guy Carpenter [Member]
Revenue	286		277		662		636
Sales Revenue, Services, Net, Originally Reported			277				636
Sales Revenue, Services, Net, Adjusted			$ 0				$ 0

[1] Includes inter-segment revenue of $3 million and $2 million in 2013 and 2012 respectively, interest income on fiduciary funds of $6 million and $10 million in 2013 and 2012, respectively, and equity method income of $7 million and $9 million in 2013 and 2012, respectively.

[2] Includes inter-segment revenue of $3 million in both 2013 and 2012, interest income on fiduciary funds of $14 million and $21 million in 2013 and 2012, respectively, and equity method income of $8 million and $10 million in 2013 and 2012, respectively.

[3] Includes inter-segment revenue of $7 million and $11 million in 2013 and 2012, respectively, and interest income on fiduciary funds of $1 million in 2013 and 2012.

[4] Includes inter-segment revenue of $14 million and $19 million in 2013 and 2012, respectively, and interest income on fiduciary funds of $2 million and $1 million in 2013 and 2012, respectively.