Segment Information (Details Of Operating Segment Revenue) (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
Sales Revenue, Services, Net, Originally Reported						3,051
Operating Income (Loss), Originally Reported						527
Sales Revenue, Services, Net, Adjusted						0
Operating Income (Loss), Adjusted						0
Marsh [Member]
Revenue	1,395		1,370	1,201	1,364	1,330		5,265
Sales Revenue, Services, Net, Originally Reported						1,388
Sales Revenue, Services, Net, Adjusted						(58)
Guy Carpenter [Member]
Revenue	376		199	250	277	359		1,085
Sales Revenue, Services, Net, Originally Reported						359
Sales Revenue, Services, Net, Adjusted						0
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
Sales Revenue, Services, Net, Originally Reported						1,747
Operating Income (Loss), Originally Reported						417
Sales Revenue, Services, Net, Adjusted						(58)
Operating Income (Loss), Adjusted						(5)
Mercer [Member]
Revenue	1,041		1,061	1,054	1,017	1,015		4,147
Sales Revenue, Services, Net, Originally Reported						957
Sales Revenue, Services, Net, Adjusted						58
Oliver Wyman Group [Member]
Revenue	321		378	351	381	356		1,466
Sales Revenue, Services, Net, Originally Reported						356
Sales Revenue, Services, Net, Adjusted						0
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
Sales Revenue, Services, Net, Originally Reported						1,313
Operating Income (Loss), Originally Reported						159
Sales Revenue, Services, Net, Adjusted						58
Operating Income (Loss), Adjusted						5
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
Sales Revenue, Services, Net, Originally Reported						3,060
Operating Income (Loss), Originally Reported						576
Sales Revenue, Services, Net, Adjusted						0
Operating Income (Loss), Adjusted						0
Corporate / Eliminations [Member]
Revenue	(7)		(6)	(11)	(13)	(9)		(39)
Operating Income (Loss)	(48)		(44)	(49)	(55)	(49)		(197)
Sales Revenue, Services, Net, Originally Reported						(9)
Operating Income (Loss), Originally Reported						(49)
Sales Revenue, Services, Net, Adjusted						0
Operating Income (Loss), Adjusted						$ 0

[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.