Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 5,017	$ 4,189	$ 10,100	$ 8,840
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	3,141	2,603	6,366	5,514
Fiduciary interest income	3	9	8	32
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	1,246	994	2,447	2,077
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	796	597	1,633	1,351
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	347	298	621	536
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	103	99	193	190
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,404	1,167	2,528	2,145
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,892	1,616	3,765	3,378
Consulting Segment | Wealth
Disaggregation of Revenue [Line Items]
Revenue	625	561	1,248	1,153
Consulting Segment | Health
Disaggregation of Revenue [Line Items]
Revenue	462	432	949	918
Consulting Segment | Career
Disaggregation of Revenue [Line Items]
Revenue	187	156	365	329
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	3,138	2,594	6,358	5,482
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,650	2,161	4,975	4,222
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	488	433	1,383	1,260
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,274	1,149	2,562	2,400
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 618	$ 467	$ 1,203	$ 978