Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 segment	Jun. 30, 2012
Segment Reporting Information
Number of reportable segments			2
Total revenue	$ 2,897	$ 2,821	$ 5,812	$ 5,662
Total commissions, fees and other revenue	2,891	2,813	5,799	5,642
Total fiduciary investment income	6	8	13	20
Operating income before income tax	382	394	792	796
Interest income	2	2	3	5
Interest expense	(48)	(57)	(100)	(116)
Other income	6	12	15	12
Income from continuing operations before income taxes	342	351	710	697
Total operating segments [Member]
Segment Reporting Information
Operating income before income tax	427	442	881	881
Risk Solutions [Member]
Segment Reporting Information
Total revenue	1,944	1,899	3,915	3,804
Total commissions, fees and other revenue	1,938	1,891	3,902	3,784
Total fiduciary investment income	6	8	13	20
Operating income before income tax	391	384	794	750
Retail brokerage
Segment Reporting Information
Total commissions, fees and other revenue	1,562	1,511	3,124	3,005
Reinsurance brokerage
Segment Reporting Information
Total commissions, fees and other revenue	376	380	778	779
HR Solutions [Member]
Segment Reporting Information
Total revenue	956	931	1,910	1,876
Total commissions, fees and other revenue	956	931	1,910	1,876
Total fiduciary investment income	0	0	0	0
Operating income before income tax	36	58	87	131
Consulting services
Segment Reporting Information
Total commissions, fees and other revenue	388	366	770	746
Outsourcing
Segment Reporting Information
Total commissions, fees and other revenue	578	570	1,159	1,138
Intrasegment
Segment Reporting Information
Total commissions, fees and other revenue	(10)	(5)	(19)	(8)
Intersegment elimination
Segment Reporting Information
Total revenue	(3)	(9)	(13)	(18)
Total commissions, fees and other revenue	(3)	(9)	(13)	(18)
Unallocated Expense
Segment Reporting Information
Operating income before income tax	$ (45)	$ (48)	$ (89)	$ (85)