Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue		$ 3,712	$ 3,504	$ 3,734	$ 4,000	$ 3,685	$ 3,341	$ 3,495	$ 3,503	$ 14,950	$ 14,024	$ 13,211
Fixed assets, net		701				712				701	712	725
United States
Segment Reporting Information [Line Items]
Revenue										7,219	6,870	6,573
Fixed assets, net		403				399				403	399	412
United Kingdom
Segment Reporting Information [Line Items]
Revenue										2,243	2,112	2,019
Fixed assets, net		91				91				91	91	94
Europe
Segment Reporting Information [Line Items]
Revenue										2,694	2,197	2,022
Fixed assets, net		59				57				59	57	53
Asia Pacific
Segment Reporting Information [Line Items]
Revenue										1,616	1,517	1,363
Fixed assets, net		74				78				74	78	76
Other Geographic Areas
Segment Reporting Information [Line Items]
Revenue										1,235	1,378	1,278
Fixed assets, net		$ 74				$ 87				74	87	90
Operating Segments
Segment Reporting Information [Line Items]
Revenue										15,007	14,074	13,255
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]									8,228	7,630	7,143
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]									6,779	6,444	6,112
Corporate and Elimination
Segment Reporting Information [Line Items]
Revenue										(57)	(50)	(44)
Marsh Insurance Group | Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue										6,923	6,433	5,997
Guy Carpenter Reinsurance Group | Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue										1,305	1,197	1,146
Mercer Consulting Group | Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue										4,732	4,528	4,323
Oliver Wyman Group Consulting Group | Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue										$ 2,047	$ 1,916	$ 1,789

[1] Includes inter-segment revenue of $6 million, $5 million and $6 million in 2018, 2017 and 2016, respectively, interest income on fiduciary funds of $65 million, $39 million and $26 million in 2018, 2017 and 2016, respectively, and equity method income of $13 million, $14 million and $12 million in 2018, 2017 and 2016, respectively and $40 million related to the sale of business in 2018.

[2] Includes inter-segment revenue of $51 million, $45 million and $38 million in 2018, 2017 and 2016, respectively, interest income on fiduciary funds of $3 million, $4 million and $3 million in 2018, 2017 and 2016, respectively, and equity method income of $8 million, $17 million and $19 million in 2018, 2017 and 2016, respectively.