Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 5,083	$ 4,651
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	3,225	2,911
Fiduciary interest income	5	23
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	1,201	1,083
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	837	754
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	274	238
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	90	91
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,124	978
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,873	1,762
Consulting Segment | Wealth
Disaggregation of Revenue [Line Items]
Revenue	623	592
Consulting Segment | Health
Disaggregation of Revenue [Line Items]
Revenue	487	486
Consulting Segment | Career
Disaggregation of Revenue [Line Items]
Revenue	178	173
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	3,220	2,888
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,325	2,061
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	895	827
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,288	1,251
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 585	$ 511