Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 2,096	$ 4,440
Fiduciary interest income	15	28
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	526	1,169
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	183	347
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	99	183
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	808	1,699
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	941	1,744
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,650	3,318
Defined Benefit Consulting & Administration | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	320	659
Investment Management & Related Services | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	232	458
Total Wealth | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	552	1,117
Health | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	429
Career | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	177
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	1,749	3,443
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	332	969
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,081	4,412
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,158	2,329
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 492	$ 989