Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 4,264	$ 3,968	$ 4,349	$ 4,071	$ 3,712	$ 3,504	$ 3,734	$ 4,000	$ 16,652	$ 14,950	$ 14,024
Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue									2,001	1,616	1,517
Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue									9,599	8,228	7,630
Fiduciary interest income									105	65	39
Risk and insurance services | Total International
Disaggregation of Revenue [Line Items]
Revenue									3,895	3,215	3,082
Risk and insurance services | EMEA
Disaggregation of Revenue [Line Items]
Revenue									2,482	2,132	2,033
Risk and insurance services | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue									953	683	645
Risk and insurance services | Latin America
Disaggregation of Revenue [Line Items]
Revenue									460	400	404
Risk and insurance services | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue									4,119	3,662	3,322
Consulting
Disaggregation of Revenue [Line Items]
Revenue									7,143	6,779	6,444
Fiduciary interest income									4	3	4
Consulting | Wealth
Disaggregation of Revenue [Line Items]
Revenue									2,369	2,185	2,148
Consulting | Health
Disaggregation of Revenue [Line Items]
Revenue									1,796	1,735	1,648
Consulting | Career
Disaggregation of Revenue [Line Items]
Revenue									856	812	732
Marsh & Guy Carpenter | Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue									9,494	8,163	7,591
Marsh | Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue									8,014	6,877	6,404
Guy Carpenter | Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue									1,480	1,286	1,187
Mercer | Consulting
Disaggregation of Revenue [Line Items]
Revenue									5,021	4,732	4,528
Oliver Wyman Group | Consulting
Disaggregation of Revenue [Line Items]
Revenue									$ 2,122	$ 2,047	$ 1,916