Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenue		$ 3,364	$ 3,135	$ 3,376	$ 3,336	$ 3,338	$ 3,115	$ 3,225	$ 3,215	$ 13,211	$ 12,893	$ 12,951
Fixed assets, net		725				773				725	773	809
United States
Segment Reporting Information [Line Items]
Revenue										6,573	6,316	5,865
Fixed assets, net		412				460				412	460	483
United Kingdom
Segment Reporting Information [Line Items]
Revenue										2,019	2,036	2,111
Fixed assets, net		94				115				94	115	120
Europe
Segment Reporting Information [Line Items]
Revenue										2,022	1,902	2,077
Fixed assets, net		53				57				53	57	60
Asia Pacific
Segment Reporting Information [Line Items]
Revenue										1,363	1,333	1,420
Fixed assets, net		76				49				76	49	62
Other Geographic Areas
Segment Reporting Information [Line Items]
Revenue										1,278	1,346	1,517
Fixed assets, net		$ 90				$ 92				90	92	84
Operating Segments
Segment Reporting Information [Line Items]
Revenue										13,255	12,933	12,990
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]									7,143	6,869	6,931
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]									6,112	6,064	6,059
Corporate and Elimination
Segment Reporting Information [Line Items]
Revenue										(44)	(40)	(39)
Marsh Insurance Group | Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue										5,997	5,745	5,774
Guy Carpenter Reinsurance Group | Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue										1,146	1,124	1,157
Mercer Consulting Group | Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue										4,323	4,313	4,350
Oliver Wyman Group Consulting Group | Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue										$ 1,789	$ 1,751	$ 1,709

[1] Includes inter-segment revenue of $6 million in both 2016 and 2015 and $4 million in 2014, interest income on fiduciary funds of $26 million, $21 million and $24 million in 2016, 2015 and 2014, respectively, and equity method income of $12 million, $6 million and $9 million in 2016, 2015 and 2014, respectively.

[2] Includes inter-segment revenue of $38 million, $34 million and $35 million in 2016, 2015 and 2014, respectively, interest income on fiduciary funds of $3 million in 2016, $4 million in 2015 and $6 million in 2014 and equity method income of $19 million in 2016, $21 million in 2015 and $2 million in 2014.