Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 4,416	$ 3,968	$ 4,189	$ 4,651	$ 4,264	$ 3,968	$ 4,349	$ 4,071	$ 17,224	$ 16,652	$ 14,950
Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue									2,093	2,001	1,616
Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue									10,337	9,599	8,228
Fiduciary interest income									46	105	65
Risk and insurance services | Total International
Disaggregation of Revenue [Line Items]
Revenue									4,058	3,895	3,215
Risk and insurance services | EMEA
Disaggregation of Revenue [Line Items]
Revenue									2,575	2,482	2,132
Risk and insurance services | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue									1,059	953	683
Risk and insurance services | Latin America
Disaggregation of Revenue [Line Items]
Revenue									424	460	400
Risk and insurance services | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue									4,537	4,119	3,662
Consulting
Disaggregation of Revenue [Line Items]
Revenue									6,976	7,143	6,779
Fiduciary interest income									1	4	3
Consulting | Wealth
Disaggregation of Revenue [Line Items]
Revenue									2,348	2,369	2,185
Consulting | Health
Disaggregation of Revenue [Line Items]
Revenue									1,793	1,796	1,735
Consulting | Career
Disaggregation of Revenue [Line Items]
Revenue									787	856	812
Marsh & Guy Carpenter | Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue									10,291	9,494	8,163
Marsh | Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue									8,595	8,014	6,877
Guy Carpenter | Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue									1,696	1,480	1,286
Mercer | Consulting
Disaggregation of Revenue [Line Items]
Revenue									4,928	5,021	4,732
Oliver Wyman Group | Consulting
Disaggregation of Revenue [Line Items]
Revenue									$ 2,048	$ 2,122	$ 2,047