Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenue	$ 3,341		$ 3,135		$ 10,339		$ 9,847
Operating Income (Loss)	597		572		2,170		2,031
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,350		3,145		10,373		9,880
Operating Income (Loss)	643		623		2,308		2,178
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1,763	[1]	1,632	[1]	5,668	[2]	5,350	[2]
Operating Income (Loss)	314		315		1,455		1,340
Interest on fiduciary funds (less than)	11		8		28		20
Equity method income	2		4		11		11
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,587	[3]	1,513	[3]	4,705	[4]	4,530	[4]
Operating Income (Loss)	329		308		853		838
Interest on fiduciary funds (less than)	2		1		3		2
Equity method income	3		5		12		14
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	(9)		(10)		(34)		(33)
Operating Income (Loss)	(46)		(51)		(138)		(147)
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1		1		5		5
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 8		$ 9		$ 29		$ 28

[1] Includes inter-segment revenue of $1 million in both 2017 and 2016, interest income on fiduciary funds of $11 million and $8 million in 2017 and 2016, respectively, and equity method income of $2 million and $4 million in 2017 and 2016, respectively.

[2] Includes inter-segment revenue of $5 million in both 2017 and 2016, interest income on fiduciary funds of $28 million and $20 million in 2017 and 2016, respectively, and equity method income of $11 million in both 2017 and 2016.

[3] Includes inter-segment revenue of $8 million and $9 million in 2017 and 2016, respectively, interest income on fiduciary funds of $2 million in 2017 and $1 million in 2016, respectively, and equity method income of $3 million in 2017 and $5 million in 2016, respectively.

[4] Includes inter-segment revenue of $29 million and $28 million in 2017 and 2016, respectively, interest income on fiduciary funds of $3 million in 2017 and $2 million in 2016, respectively, and equity method income of $12 million in 2017 and $14 million in 2016, respectively.