Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Revenue	$ 4,189	$ 4,349	$ 8,840	$ 8,420
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,603	2,574	5,514	4,997
Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,161	2,156	4,222	3,893
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	433	392	1,260	1,055
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,616	1,800	3,378	3,473
Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,149	1,260	2,400	2,415
Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	467	540	978	1,058
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,219	4,374	8,892	8,470
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,603	2,574	5,514	4,997
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,168	2,174	4,244	3,927
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	435	400	1,270	1,070
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,616	1,800	3,378	3,473
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,149	1,260	2,400	2,415
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	467	540	978	1,058
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (30)	$ (25)	$ (52)	$ (50)