Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenue	$ 19,820	$ 17,224	$ 16,652
Fixed Assets, Net	847	856	858
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	12,085	10,337	9,599
Consulting
Segment Reporting Information [Line Items]
Revenue	7,789	6,976	7,143
United States
Segment Reporting Information [Line Items]
Revenue	9,343	8,168	7,840
Fixed Assets, Net	484	492	462
United Kingdom
Segment Reporting Information [Line Items]
Revenue	3,130	2,818	2,679
Fixed Assets, Net	116	115	149
Continental Europe
Segment Reporting Information [Line Items]
Revenue	3,219	2,881	2,837
Fixed Assets, Net	68	74	68
Asia Pacific
Segment Reporting Information [Line Items]
Revenue	2,617	2,093	2,001
Fixed Assets, Net	96	105	101
Asia Pacific | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	1,462	1,059	953
Other
Segment Reporting Information [Line Items]
Revenue	1,565	1,353	1,385
Fixed Assets, Net	83	70	78
Operating Segments
Segment Reporting Information [Line Items]
Revenue	19,874	17,313	16,742
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	12,085	10,337	9,599
Gain (loss) on consolidation of business	267
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	7,789	6,976	7,143
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(54)	(89)	(90)
Marsh | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	10,203	8,595	8,014
Marsh | Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	10,214	8,628	8,085
Guy Carpenter | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	1,867	1,696	1,480
Guy Carpenter | Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	1,871	1,709	1,514
Mercer | Consulting
Segment Reporting Information [Line Items]
Revenue	5,254	4,928	5,021
Mercer | Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	5,254	4,928	5,021
Oliver Wyman Group | Consulting
Segment Reporting Information [Line Items]
Revenue	2,535	2,048	2,122
Oliver Wyman Group | Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	$ 2,535	$ 2,048	$ 2,122