Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 segment	Sep. 30, 2012
Segment Reporting Information
Number of reportable segments			2
Total revenue	$ 2,794	$ 2,737	$ 8,606	$ 8,399
Total commissions, fees and other revenue	2,786	2,726	8,585	8,368
Total fiduciary investment income	8	11	21	31
Operating income before income tax	364	339	1,156	1,135
Interest income	3	1	6	6
Interest expense	(53)	(57)	(153)	(173)
Other income (expense)	39	(9)	54	3
Income from continuing operations before income taxes	353	274	1,063	971
Total operating segments [Member]
Segment Reporting Information
Operating income before income tax	408	389	1,289	1,270
Risk Solutions [Member]
Segment Reporting Information
Total revenue	1,821	1,778	5,736	5,582
Total commissions, fees and other revenue	1,813	1,767	5,715	5,551
Total fiduciary investment income	8	11	21	31
Operating income before income tax	333	309	1,127	1,059
Retail brokerage
Segment Reporting Information
Total commissions, fees and other revenue	1,424	1,390	4,548	4,395
Reinsurance brokerage
Segment Reporting Information
Total commissions, fees and other revenue	389	377	1,167	1,156
HR Solutions [Member]
Segment Reporting Information
Total revenue	981	971	2,891	2,847
Total commissions, fees and other revenue	981	971	2,891	2,847
Total fiduciary investment income	0	0	0	0
Operating income before income tax	75	80	162	211
Consulting services
Segment Reporting Information
Total commissions, fees and other revenue	406	394	1,176	1,140
Outsourcing
Segment Reporting Information
Total commissions, fees and other revenue	587	585	1,746	1,723
Intrasegment
Segment Reporting Information
Total commissions, fees and other revenue	(12)	(8)	(31)	(16)
Intersegment elimination
Segment Reporting Information
Total revenue	(8)	(12)	(21)	(30)
Total commissions, fees and other revenue	(8)	(12)	(21)	(30)
Unallocated Expense
Segment Reporting Information
Operating income before income tax	$ (44)	$ (50)	$ (133)	$ (135)