Revenue - Disaggregation of Revenue (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 2,423
Fiduciary interest income	23
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	633
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	165
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	78
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	876
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	861
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,673
Total Wealth | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	543
Health | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	442
Career | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	170
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	1,737
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	663
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,400
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,155
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 518