Segment Information (Selected Information and Details Of Operating Segment 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
Segment Reporting Information [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
Fixed Assets, Net	858				701				858	701	712
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									9,599	8,228	7,630
Consulting
Segment Reporting Information [Line Items]
Revenue									7,143	6,779	6,444
United States
Segment Reporting Information [Line Items]
Revenue									7,840	7,219	6,870
Fixed Assets, Net	462				403				462	403	399
United Kingdom
Segment Reporting Information [Line Items]
Revenue									2,679	2,243	2,112
Fixed Assets, Net	149				91				149	91	91
Continental Europe
Segment Reporting Information [Line Items]
Revenue									2,837	2,694	2,197
Fixed Assets, Net	68				59				68	59	57
Asia Pacific
Segment Reporting Information [Line Items]
Revenue									2,001	1,616	1,517
Fixed Assets, Net	101				74				101	74	78
Asia Pacific | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									953	683	645
Other
Segment Reporting Information [Line Items]
Revenue									1,385	1,235	1,378
Fixed Assets, Net	$ 78				$ 74				78	74	87
Operating Segments
Segment Reporting Information [Line Items]
Revenue									16,742	15,007	14,074
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									9,599	8,228	7,630
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue									7,143	6,779	6,444
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue									(90)	(57)	(50)
Marsh | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									8,014	6,877	6,404
Marsh | Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									8,085	6,923	6,433
Guy Carpenter | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									1,480	1,286	1,187
Guy Carpenter | Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue									1,514	1,305	1,197
Mercer | Consulting
Segment Reporting Information [Line Items]
Revenue									5,021	4,732	4,528
Mercer | Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue									5,021	4,732	4,528
Oliver Wyman Group | Consulting
Segment Reporting Information [Line Items]
Revenue									2,122	2,047	1,916
Oliver Wyman Group | Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue									$ 2,122	$ 2,047	$ 1,916