Segment Information (Details of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment Reporting Information [Line Items]
Revenue	$ 3,141		$ 2,932		$ 9,705		$ 9,146
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	3,151		2,941		9,733		9,172
Operating Segments [Member] | Marsh Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,343		1,241		4,295		4,038
Operating Segments [Member] | Guy Carpenter Reinsurance Group [Member]
Segment Reporting Information [Line Items]
Revenue	267		263		945		925
Operating Segments [Member] | Mercer Consulting Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,112		1,072		3,244		3,157
Operating Segments [Member] | Oliver Wyman Group Consulting Group [Member]
Segment Reporting Information [Line Items]
Revenue	429		365		1,249		1,052
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,610	[1]	1,504	[1]	5,240	[2]	4,963	[2]
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,541	[3]	1,437	[3]	4,493	[4]	4,209	[4]
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(10)		(9)		(28)		(26)
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	0		1		3		4
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	$ 10		$ 8		$ 25		$ 22

[1] Includes inter-segment revenue of $0 million and $1 million in 2014 and 2013, respectively, interest income on fiduciary funds of $6 million and $7 million in 2014 and 2013, respectively, and equity method income of $4 million and $2 million in 2014 and 2013, respectively.

[2] Includes inter-segment revenue of $3 million and $4 million in 2014 and 2013, respectively, interest income on fiduciary funds of $18 million and $21 million in 2014 and 2013, respectively, and equity method income of $10 million in both 2014 and 2013.

[3] Includes inter-segment revenue of $10 million and $8 million in 2014 and 2013, respectively, and interest income on fiduciary funds of $2 million in 2014 and $1 million in 2013.

[4] Includes inter-segment revenue of $25 million and $22 million in 2014 and 2013, respectively, and interest income on fiduciary funds of $5 million and $3 million in 2014 and 2013, respectively.