Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 2,574	$ 4,997
Fiduciary interest income	26	49
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	652	1,285
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	291	456
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	116	194
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	1,059	1,935
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,097	1,958
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,800	3,473
Wealth | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	613	1,156
Health | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	458	900
Career | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	189	359
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,156	3,893
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	392	1,055
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,548	4,948
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,260	2,415
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 540	$ 1,058