Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended							12 Months Ended
	Mar. 31, 2013		Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012		Dec. 31, 2012
Revenue	$ 3,126		$ 3,002	$ 2,845	$ 3,026	$ 3,051		$ 11,924
Operating Income (Loss)	607		406	378	518	527		1,829
Interest income on fiduciary funds	4					6
Equity method income	20					20
Risk and Insurance Services [Member]
Revenue	1,771	[1]	1,569	1,451	1,641	1,689	[1]	6,350
Operating Income (Loss)	468		310	222	390	412		1,334
Interest income on fiduciary funds	8					11
Equity method income	1					1
Consulting [Member]
Revenue	1,362	[2]	1,439	1,405	1,398	1,371	[2]	5,613
Operating Income (Loss)	187		140	205	183	164		692
Inter-segment revenue	7					8
Interest income on fiduciary funds	1					1
Total Operating Segments [Member]
Revenue	3,133		3,008	2,856	3,039	3,060		11,963
Operating Income (Loss)	655		450	427	573	576		2,026
Corporate / Eliminations [Member]
Revenue	(7)		(6)	(11)	(13)	(9)		(39)
Operating Income (Loss)	(48)		(44)	(49)	(55)	(49)		(197)
Risk and Insurance Services [Member]
Inter-segment revenue						$ 1

[1] Includes inter-segment revenue of $1 million in 2012, interest income on fiduciary funds of $8 million and $11 million in 2013 and 2012, respectively, and equity method income of $1 million in both 2013 and 2012, respectively.

[2] Includes inter-segment revenue of $7 million and $8 million in 2013 and 2012, respectively, and interest income on fiduciary funds of $1 million in both 2013 and 2012 respectively.