Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenue	$ 3,246	$ 3,141	$ 3,300	$ 3,264	$ 3,115	$ 2,932	$ 3,088	$ 3,126	$ 12,951		$ 12,261		$ 11,924
Fixed Assets, Net	809				828				809		828		809
United States [Member]
Segment Reporting Information [Line Items]
Revenue									5,865		5,485		5,300
Fixed Assets, Net	483				494				483		494		494
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenue									2,111		1,979		1,960
Fixed Assets, Net	120				121				120		121		121
Europe [Member]
Segment Reporting Information [Line Items]
Revenue									2,077		1,943		1,879
Fixed Assets, Net	60				64				60		64		63
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Revenue									1,420		1,396		1,346
Fixed Assets, Net	62				72				62		72		62
Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Revenue									1,517		1,494		1,478
Fixed Assets, Net	84				77				84		77		69
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue									12,990		12,297		11,963
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue									6,931	[1]	6,596	[1]	6,350	[1]
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue									6,059	[2]	5,701	[2]	5,613	[2]
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue									(39)		(36)		(39)
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue									4		5		5
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue									35		31		34
Marsh Insurance Group [Member] | Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue									5,774		5,461		5,265
Guy Carpenter Reinsurance Group [Member] | Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue									1,157		1,135		1,085
Mercer Consulting Group [Member] | Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue									4,350		4,241		4,147
Oliver Wyman Group Consulting Group [Member] | Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue									$ 1,709		$ 1,460		$ 1,466

[1] Includes inter-segment revenue of $4 million in 2014 and $5 million in both 2013 and 2012, interest income on fiduciary funds of $24 million, $27 million and $39 million in 2014, 2013 and 2012, respectively, and equity method income of $9 million, $8 million and $11 million in 2014, 2013 and 2012, respectively.

[2] Includes inter-segment revenue of $35 million, $31 million and $34 million in 2014, 2013 and 2012, respectively, interest income on fiduciary funds of $6 million in 2014, $5 million in 2013 and $4 million 2012 and equity method income of $2 million in 2014, and $0 million in both 2013 and 2012.