Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 4,651	$ 4,071
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,911
Fiduciary interest income	23
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	754
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	238
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	91
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	1,083
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	978
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,762
Consulting Segment | Wealth
Disaggregation of Revenue [Line Items]
Revenue	592
Consulting Segment | Health
Disaggregation of Revenue [Line Items]
Revenue	486
Consulting Segment | Career
Disaggregation of Revenue [Line Items]
Revenue	173
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,061
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	827
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,888
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,251
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 511