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Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Revenue	$ 5,083	$ 4,651
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,225	2,911
Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,325	2,061
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	895	827
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,873	1,762
Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,288	1,251
Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	585	511
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,098	4,673
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,225	2,911
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,329	2,076
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	896	835
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,873	1,762
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,288	1,251
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	585	511
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (15)	$ (22)

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