Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015
Segment Reporting Information [Line Items]
Revenue	$ 3,135		$ 3,115		$ 9,847		$ 9,555
Operating Income (Loss)	572		461		2,031		1,825
Interest on fiduciary funds (less than)	8		6		20		16
Consulting Segment
Segment Reporting Information [Line Items]
Interest on fiduciary funds (less than)	1		1		2		3
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,145		3,124		9,880		9,585
Operating Income (Loss)	623		510		2,178		1,966
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1,632	[1]	1,584	[1]	5,350	[2]	5,137	[2]
Operating Income (Loss)	315		225		1,340		1,185
Interest on fiduciary funds (less than)	8		6		20	[2]	16	[2]
Equity method income	4		2		11	[2]	4	[2]
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,513	[3]	1,540	[3]	4,530	[4]	4,448	[4]
Operating Income (Loss)	308		285		838		781
Interest on fiduciary funds (less than)	1		1		2		3
Equity method income	5		5		14	[4]	13
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	(10)		(9)		(33)		(30)
Operating Income (Loss)	(51)		(49)		(147)		(141)
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1		0		5	[2]	5	[2]
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 9		$ 9		$ 28		$ 25

[1] Includes inter-segment revenue of $1 million and $0 million in 2016 and 2015, respectively, interest income on fiduciary funds of $8 million and $6 million in 2016 and 2015, respectively, and equity method income of $4 million in 2016 and $2 million in 2015, respectively.

[2] Includes inter-segment revenue of $5 million in both 2016 and in 2015, interest income on fiduciary funds of $20 million and $16 million in 2016 and 2015, respectively, and equity method income of $11 million and $4 million in 2016 and 2015, respectively.

[3] Includes inter-segment revenue of $9 million in both 2016 and 2015, interest income on fiduciary funds of $1 million in both 2016 and 2015, and equity method income of $5 million in both 2016 and 2015.

[4] Includes inter-segment revenue of $28 million and $25 million in 2016 and 2015, respectively, interest income on fiduciary funds of $2 million and $3 million in 2016 and 2015, respectively, and equity method income of $14 million and $13 million in 2016 and 2015, respectively.