Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 4,189	$ 4,349	$ 8,840	$ 8,420
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,603	2,574	5,514	4,997
Fiduciary interest income	9	26	32	49
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	597	652	1,351	1,285
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	298	291	536	456
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	99	116	190	194
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	994	1,059	2,077	1,935
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,167	1,097	2,145	1,958
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,616	1,800	3,378	3,473
Consulting Segment | Wealth
Disaggregation of Revenue [Line Items]
Revenue	561	613	1,153	1,156
Consulting Segment | Health
Disaggregation of Revenue [Line Items]
Revenue	432	458	918	900
Consulting Segment | Career
Disaggregation of Revenue [Line Items]
Revenue	156	189	329	359
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,161	2,156	4,222	3,893
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	433	392	1,260	1,055
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,594	2,548	5,482	4,948
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,149	1,260	2,400	2,415
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 467	$ 540	$ 978	$ 1,058