Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Revenue	$ 4,651	$ 4,071
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,911
Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,061
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	827
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,762
Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,251
Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	511
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,673	4,096
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,911	2,423
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,076	1,753
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	835	670
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,762	1,673
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,251	1,155
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	511	518
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (22)	$ (25)