Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenue	$ 4,000	$ 3,503
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,012	3,515
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,344	1,989
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	1,703	1,602
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	641	387
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,668	1,526
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,171	1,077
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	497	449
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (12)	$ (12)