Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2015		Jun. 30, 2014		Jun. 30, 2015		Jun. 30, 2014
Segment Reporting Information [Line Items]
Revenue	$ 3,225		$ 3,300		$ 6,440		$ 6,564
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	3,237		3,311		6,461		6,582
Operating Segments [Member] | Marsh Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,474		1,495		2,908		2,952
Operating Segments [Member] | Guy Carpenter Reinsurance Group [Member]
Segment Reporting Information [Line Items]
Revenue	276		296		645		678
Operating Segments [Member] | Mercer Consulting Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,046		1,071		2,083		2,132
Operating Segments [Member] | Oliver Wyman Group Consulting Group [Member]
Segment Reporting Information [Line Items]
Revenue	441		449		825		820
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,750	[1]	1,791	[1]	3,553	[2]	3,630	[2]
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,487	[3]	1,520	[3]	2,908	[4]	2,952	[4]
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(12)		(11)		(21)		(18)
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	4		3		5		3
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	$ 8		$ 8		$ 16		$ 15

[1] Includes inter-segment revenue of $4 million and $3 million in 2015 and 2014, respectively, interest income on fiduciary funds of $5 million and $6 million in 2015 and 2014, respectively, and equity method income of $0 million and $6 million in 2015 and 2014, respectively.

[2] Includes inter-segment revenue of $5 million and $3 million in 2015 and 2014, respectively, interest income on fiduciary funds of $10 million and $12 million in 2015 and 2014, respectively, and equity method income of $2 million in 2015 and $6 million in 2014.

[3] Includes inter-segment revenue of $8 million in both 2015 and 2014, interest income on fiduciary funds of $1 million in 2015 and $2 million in 2014, and equity method income of $5 million in 2015 and $0 million in 2014.

[4] Includes inter-segment revenue of $16 million and $15 million in 2015 and 2014, respectively, interest income on fiduciary funds of $2 million and $3 million in 2015 and 2014, respectively, and equity method income of $8 million in 2015 and $0 million in 2014.