Segment Information (Selected Information and Details Of Operating Segment 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
Segment Reporting Information [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
Fixed Assets, Net	856				858				856	858	701
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									10,337	9,599	8,228
Consulting
Segment Reporting Information [Line Items]
Revenue									6,976	7,143	6,779
United States
Segment Reporting Information [Line Items]
Revenue									8,168	7,840	7,219
Fixed Assets, Net	492				462				492	462	403
United Kingdom
Segment Reporting Information [Line Items]
Revenue									2,818	2,679	2,243
Fixed Assets, Net	115				149				115	149	91
Continental Europe
Segment Reporting Information [Line Items]
Revenue									2,881	2,837	2,694
Fixed Assets, Net	74				68				74	68	59
Asia Pacific
Segment Reporting Information [Line Items]
Revenue									2,093	2,001	1,616
Fixed Assets, Net	105				101				105	101	74
Asia Pacific | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									1,059	953	683
Other
Segment Reporting Information [Line Items]
Revenue									1,353	1,385	1,235
Fixed Assets, Net	$ 70				$ 78				70	78	74
Operating Segments
Segment Reporting Information [Line Items]
Revenue									17,313	16,742	15,007
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									10,337	9,599	8,228
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue									6,976	7,143	6,779
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue									(89)	(90)	(57)
Marsh | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									8,595	8,014	6,877
Marsh | Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									8,628	8,085	6,923
Guy Carpenter | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									1,696	1,480	1,286
Guy Carpenter | Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									1,709	1,514	1,305
Mercer | Consulting
Segment Reporting Information [Line Items]
Revenue									4,928	5,021	4,732
Mercer | Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue									4,928	5,021	4,732
Oliver Wyman Group | Consulting
Segment Reporting Information [Line Items]
Revenue									2,048	2,122	2,047
Oliver Wyman Group | Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue									$ 2,048	$ 2,122	$ 2,047