Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenue		$ 3,338	$ 3,115	$ 3,225	$ 3,215	$ 3,246	$ 3,141	$ 3,300	$ 3,264	$ 12,893	$ 12,951	$ 12,261
Fixed assets, net		773				809				773	809	828
United States
Segment Reporting Information [Line Items]
Revenue										6,316	5,865	5,485
Fixed assets, net		460				483				460	483	494
United Kingdom
Segment Reporting Information [Line Items]
Revenue										2,036	2,111	1,979
Fixed assets, net		115				120				115	120	121
Europe
Segment Reporting Information [Line Items]
Revenue										1,902	2,077	1,943
Fixed assets, net		57				60				57	60	64
Asia Pacific
Segment Reporting Information [Line Items]
Revenue										1,333	1,420	1,396
Fixed assets, net		49				62				49	62	72
Other Geographic Areas
Segment Reporting Information [Line Items]
Revenue										1,346	1,517	1,494
Fixed assets, net		$ 92				$ 84				92	84	77
Operating Segments
Segment Reporting Information [Line Items]
Revenue										12,933	12,990	12,297
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]									6,869	6,931	6,596
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]									6,064	6,059	5,701
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue										(40)	(39)	(36)
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue										6	4	5
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue										34	35	31
Marsh Insurance Group | Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue										5,745	5,774	5,461
Guy Carpenter Reinsurance Group | Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue										1,124	1,157	1,135
Mercer Consulting Group | Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue										4,313	4,350	4,241
Oliver Wyman Group Consulting Group | Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue										$ 1,751	$ 1,709	$ 1,460

[1] Includes inter-segment revenue of $6 million in 2015, $4 million in 2014 and $5 million in 2013, interest income on fiduciary funds of $21 million, $24 million and $27 million in 2015, 2014 and 2013, respectively, and equity method income of $6 million, $9 million and $8 million in 2015, 2014 and 2013, respectively.

[2] Includes inter-segment revenue of $34 million, $35 million and $31 million in 2015, 2014 and 2013, respectively, interest income on fiduciary funds of $4 million in 2015, $6 million in 2014 and $5 million in 2013 and equity method income of $21 million in 2015, $2 million in 2014 and $0 million in 2013.