Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2016		Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Segment Reporting Information [Line Items]
Revenue	$ 3,376		$ 3,225		$ 6,712		$ 6,440
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3		4		4	[1]	5	[1]
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	10		8		19		16
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,389		3,237		6,735		6,461
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1,850	[2]	1,750	[2]	3,718	[1]	3,553	[1]
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	1,564		1,474		3,057		2,908
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	286		276		661		645
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,539	[3]	1,487	[3]	3,017	[4]	2,908	[4]
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,079		1,046		2,118		2,083
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	460		441		899		825
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (13)		$ (12)		$ (23)		$ (21)

[1] Includes inter-segment revenue of $4 million in 2016 and $5 million in 2015, interest income on fiduciary funds of $12 million and $10 million in 2016 and 2015, respectively, and equity method income of $7 million and $2 million in 2016 and 2015, respectively.

[2] Includes inter-segment revenue of $3 million and $4 million in 2016 and 2015, respectively, interest income on fiduciary funds of $6 million and $5 million in 2016 and 2015, respectively, and equity method income of $6 million in 2016 and $0 million in 2015, respectively.

[3] Includes inter-segment revenue of $10 million and $8 million in 2016 and 2015, respectively, interest income on fiduciary funds of less than $1 million and $1 million in 2016 and 2015, respectively, and equity method income of $5 million in both 2016 and 2015.

[4] Includes inter-segment revenue of $19 million and $16 million in 2016 and 2015, respectively, interest income on fiduciary funds of $1 million and $2 million in 2016 and 2015, respectively, and equity method income of $9 million and $8 million in 2016 and 2015, respectively.