Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenue	$ 4,349	$ 3,734	$ 8,420	$ 7,734
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,374	3,746	8,470	7,758
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,574	2,096	4,997	4,440
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,174	1,760	3,927	3,463
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	400	336	1,070	977
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,800	1,650	3,473	3,318
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,260	1,158	2,415	2,329
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	540	492	1,058	989
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (25)	$ (12)	$ (50)	$ (24)