Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 3,968	$ 3,968	$ 12,808	$ 12,388
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,291	2,206	7,805	7,203
Fiduciary interest income	8	31	40	80
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	883	888	2,960	2,823
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	536	536	1,887	1,821
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	254	242	790	698
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	93	110	283	304
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,126	1,014	3,271	2,972
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,696	1,785	5,074	5,258
Consulting Segment | Wealth
Disaggregation of Revenue [Line Items]
Revenue	566	592	1,719	1,748
Consulting Segment | Health
Disaggregation of Revenue [Line Items]
Revenue	430	441	1,348	1,341
Consulting Segment | Career
Disaggregation of Revenue [Line Items]
Revenue	220	247	549	606
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,283	2,175	7,765	7,123
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,009	1,902	6,231	5,795
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	274	273	1,534	1,328
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,216	1,280	3,616	3,695
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 480	$ 505	$ 1,458	$ 1,563