Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 2,206	$ 7,203
Fiduciary interest income	31	80
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	536	1,821
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	242	698
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	110	304
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	888	2,823
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,014	2,972
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,785	5,258
Wealth | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	592	1,748
Health | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	441	1,341
Career | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	247	606
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	1,902	5,795
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	273	1,328
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,175	7,123
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,280	3,695
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 505	$ 1,563