Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 segment	Jun. 30, 2013
Segment Reporting Information
Number of reportable segments			2
Total revenue	$ 2,919	$ 2,897	$ 5,866	$ 5,812
Total commissions, fees and other revenue	2,913	2,891	5,854	5,799
Total fiduciary investment income	6	6	12	13
Operating income before income tax	445	382	914	792
Interest income	2	2	4	3
Interest expense	(65)	(48)	(123)	(100)
Other (expense) income	(2)	6	(1)	15
Income before income taxes	380	342	794	710
Total operating segments [Member]
Segment Reporting Information
Operating income before income tax	486	427	998	881
Risk Solutions [Member]
Segment Reporting Information
Total revenue	1,948	1,944	3,942	3,915
Total commissions, fees and other revenue	1,942	1,938	3,930	3,902
Total fiduciary investment income	6	6	12	13
Operating income before income tax	417	391	862	794
Retail brokerage
Segment Reporting Information
Total commissions, fees and other revenue	1,582	1,562	3,161	3,124
Reinsurance brokerage
Segment Reporting Information
Total commissions, fees and other revenue	360	376	769	778
HR Solutions [Member]
Segment Reporting Information
Total revenue	982	956	1,947	1,910
Total commissions, fees and other revenue	982	956	1,947	1,910
Total fiduciary investment income	0	0	0	0
Operating income before income tax	69	36	136	87
Consulting services
Segment Reporting Information
Total commissions, fees and other revenue	395	388	779	770
Outsourcing
Segment Reporting Information
Total commissions, fees and other revenue	595	578	1,184	1,159
Intrasegment
Segment Reporting Information
Total commissions, fees and other revenue	(8)	(10)	(16)	(19)
Intersegment elimination
Segment Reporting Information
Total revenue	(11)	(3)	(23)	(13)
Total commissions, fees and other revenue	(11)	(3)	(23)	(13)
Unallocated Expense
Segment Reporting Information
Operating income before income tax	$ (41)	$ (45)	$ (84)	$ (89)