Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 8,228	$ 7,630
Interest income, fiduciary assets	65	39	$ 26
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	2,132	2,033
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	683	645
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	400	404
Risk and Insurance Services Segment | International
Disaggregation of Revenue [Line Items]
Revenue	3,215	3,082
Risk and Insurance Services Segment | Americas
Disaggregation of Revenue [Line Items]
Revenue	3,662	3,322
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	6,779	6,444
Interest income, fiduciary assets	3	4	$ 3
Consulting Segment | Defined Benefit Consulting & Administration
Disaggregation of Revenue [Line Items]
Revenue	1,279	1,381
Consulting Segment | Investment Management & Related Services
Disaggregation of Revenue [Line Items]
Revenue	906	767
Consulting Segment | Total Wealth
Disaggregation of Revenue [Line Items]
Revenue	2,185	2,148
Consulting Segment | Health
Disaggregation of Revenue [Line Items]
Revenue	1,735	1,648
Consulting Segment | Career
Disaggregation of Revenue [Line Items]
Revenue	812	732
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	6,877	6,404
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	1,286	1,187
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	8,163	7,591
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	4,732	4,528
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 2,047	$ 1,916