Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) (USD $)	3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended	12 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Risk and Insurance Services Segment [Member]	Sep. 30, 2012 Risk and Insurance Services Segment [Member]	Sep. 30, 2013 Consulting Segment [Member]	Sep. 30, 2012 Consulting Segment [Member]	Sep. 30, 2013 Consulting Segment [Member]	Sep. 30, 2012 Consulting Segment [Member]	Sep. 30, 2013 Operating Segments [Member]	Sep. 30, 2012 Operating Segments [Member]	Sep. 30, 2013 Operating Segments [Member]	Sep. 30, 2012 Operating Segments [Member]	Sep. 30, 2013 Operating Segments [Member] Risk and Insurance Services Segment [Member]		Sep. 30, 2012 Operating Segments [Member] Risk and Insurance Services Segment [Member]		Sep. 30, 2013 Operating Segments [Member] Risk and Insurance Services Segment [Member]		Sep. 30, 2012 Operating Segments [Member] Risk and Insurance Services Segment [Member]		Sep. 30, 2013 Operating Segments [Member] Consulting Segment [Member]		Sep. 30, 2012 Operating Segments [Member] Consulting Segment [Member]		Sep. 30, 2013 Operating Segments [Member] Consulting Segment [Member]		Sep. 30, 2012 Operating Segments [Member] Consulting Segment [Member]		Sep. 30, 2013 Intersegment Eliminations [Member]	Sep. 30, 2012 Intersegment Eliminations [Member]	Sep. 30, 2013 Intersegment Eliminations [Member]	Sep. 30, 2012 Intersegment Eliminations [Member]	Sep. 30, 2013 Intersegment Eliminations [Member] Risk and Insurance Services Segment [Member]	Sep. 30, 2012 Intersegment Eliminations [Member] Risk and Insurance Services Segment [Member]	Sep. 30, 2013 Intersegment Eliminations [Member] Risk and Insurance Services Segment [Member]	Dec. 31, 2012 Intersegment Eliminations [Member] Risk and Insurance Services Segment [Member]	Sep. 30, 2013 Intersegment Eliminations [Member] Consulting Segment [Member]	Sep. 30, 2012 Intersegment Eliminations [Member] Consulting Segment [Member]	Sep. 30, 2013 Intersegment Eliminations [Member] Consulting Segment [Member]	Sep. 30, 2012 Intersegment Eliminations [Member] Consulting Segment [Member]	Sep. 30, 2013 Corporate, Non-Segment [Member]	Sep. 30, 2012 Corporate, Non-Segment [Member]	Sep. 30, 2013 Corporate, Non-Segment [Member]	Sep. 30, 2012 Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenue	$ 2,932,000,000	$ 2,845,000,000	$ 9,146,000,000	$ 8,922,000,000							$ 2,941,000,000	$ 2,856,000,000	$ 9,172,000,000	$ 8,955,000,000	$ 1,504,000,000	[1]	$ 1,451,000,000	[1]	$ 4,963,000,000	[2]	$ 4,781,000,000	[2]	$ 1,437,000,000	[3]	$ 1,405,000,000	[3]	$ 4,209,000,000	[4]	$ 4,174,000,000	[4]	$ (9,000,000)	$ (11,000,000)	$ (26,000,000)	$ (33,000,000)	$ 1,000,000	$ 1,000,000	$ 4,000,000	$ 4,000,000	$ 8,000,000	$ 10,000,000	$ 22,000,000	$ 29,000,000
Operating Income (Loss)	404,000,000	378,000,000	1,588,000,000	1,423,000,000							454,000,000	427,000,000	1,735,000,000	1,576,000,000	222,000,000		222,000,000		1,111,000,000		1,024,000,000		232,000,000		205,000,000		624,000,000		552,000,000														(50,000,000)	(49,000,000)	(147,000,000)	(153,000,000)
Interest income on fiduciary funds					21,000,000	31,000,000	1,000,000	1,000,000	3,000,000	2,000,000					7,000,000		10,000,000		21,000,000		31,000,000						3,000,000		2,000,000
Equity method income															$ 2,000,000				$ 10,000,000		$ 10,000,000

[1] Includes inter-segment revenue of $1 million in both 2013 and 2012, interest income on fiduciary funds of $7 million and $10 million in 2013 and 2012, respectively, and equity method income of $2 million in 2013.

[2] Includes inter-segment revenue of $4 million in both 2013 and 2012, interest income on fiduciary funds of $21 million and $31 million in 2013 and 2012, respectively, and equity method income of $10 million in both 2013 and 2012.

[3] Includes inter-segment revenue of $8 million and $10 million in 2013 and 2012, respectively, and interest income on fiduciary funds of $1 million in both 2013 and 2012.

[4] Includes inter-segment revenue of $22 million and $29 million in 2013 and 2012, respectively, and interest income on fiduciary funds of $3 million and $2 million in 2013 and 2012, respectively.