Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 segment	Sep. 30, 2013
Segment Reporting Information
Number of reportable segments			2
Total revenue	$ 2,880	$ 2,794	$ 8,746	$ 8,606
Total commissions, fees and other revenue	2,873	2,786	8,727	8,585
Total fiduciary investment income	7	8	19	21
Operating income before income tax	417	364	1,331	1,156
Interest income	3	3	7	6
Interest expense	(65)	(53)	(188)	(153)
Other income	35	39	34	54
Income before income taxes	390	353	1,184	1,063
Total operating segments [Member]
Segment Reporting Information
Operating income before income tax	456	408	1,454	1,289
Risk Solutions [Member]
Segment Reporting Information
Total revenue	1,836	1,821	5,778	5,736
Total commissions, fees and other revenue	1,829	1,813	5,759	5,715
Total fiduciary investment income	7	8	19	21
Operating income before income tax	343	333	1,205	1,127
Retail brokerage [Member]
Segment Reporting Information
Total commissions, fees and other revenue	1,458	1,424	4,619	4,548
Reinsurance brokerage [Member]
Segment Reporting Information
Total commissions, fees and other revenue	371	389	1,140	1,167
HR Solutions [Member]
Segment Reporting Information
Total revenue	1,057	981	3,004	2,891
Total commissions, fees and other revenue	1,057	981	3,004	2,891
Total fiduciary investment income	0	0	0	0
Operating income before income tax	113	75	249	162
Consulting services [Member]
Segment Reporting Information
Total commissions, fees and other revenue	466	406	1,245	1,176
Outsourcing [Member]
Segment Reporting Information
Total commissions, fees and other revenue	604	587	1,788	1,746
Intrasegment [Member]
Segment Reporting Information
Total commissions, fees and other revenue	(13)	(12)	(29)	(31)
Intersegment elimination [Member]
Segment Reporting Information
Total revenue	(13)	(8)	(36)	(21)
Total commissions, fees and other revenue	(13)	(8)	(36)	(21)
Unallocated Expense [Member]
Segment Reporting Information
Operating income before income tax	$ (39)	$ (44)	$ (123)	$ (133)