Revenue - Disaggregation of Revenue (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2,344
Fiduciary interest income	13
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	643
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	164
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	84
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	891
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	803
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,668
Defined Benefit Consulting & Administration | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	339
Investment Management & Related Services | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	226
Total Wealth | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	565
Health | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	442
Career | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	164
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,694
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	637
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2,331
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,171
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 497