Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 1,863	$ 6,303
Fiduciary interest income	18	46
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	441	1,610
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	167	514
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	96	279
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	704	2,403
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	926	2,670
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,656	4,974
Defined Benefit Consulting & Administration | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	300	959
Investment Management & Related Services | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	225	683
Total Wealth | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	525	1,642
Health | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	415	1,286
Career | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	235	576
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	1,630	5,073
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	215	1,184
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	1,845	6,257
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,175	3,504
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 481	$ 1,470