Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenue	$ 3,734	$ 3,495	$ 7,734	$ 6,998
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,746	3,508	7,758	7,023
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,096	1,916	4,440	3,905
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	1,760	1,621	3,463	3,223
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	336	295	977	682
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,650	1,592	3,318	3,118
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,158	1,109	2,329	2,186
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	492	483	989	932
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (12)	$ (13)	$ (24)	$ (25)