Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenue	$ 3,504	$ 3,341	$ 11,238	$ 10,339
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,519	3,350	11,277	10,373
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1,863	1,763	6,303	5,668
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	1,643	1,490	5,106	4,713
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	220	273	1,197	955
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,656	1,587	4,974	4,705
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,175	1,149	3,504	3,335
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	481	438	1,470	1,370
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (15)	$ (9)	$ (39)	$ (34)