Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 item	Jun. 30, 2011
Segment Information
Number of reportable segments			2
Segment Reporting Information
Revenue	$ 2,821	$ 2,811	$ 5,662	$ 5,570
Commissions, fees and other revenues	2,813	2,799	5,642	5,547
Fiduciary investment income	8	12	20	23
Operating income from continuing operations before income taxes	394	440	796	838
Interest income	2	4	5	10
Interest expense	(57)	(63)	(116)	(126)
Other income (expense)	12	(29)	12	(14)
Income from continuing operations before income taxes	351	352	697	708
Total operating segments
Segment Reporting Information
Operating income from continuing operations before income taxes	442	473	881	903
Risk Solutions
Segment Reporting Information
Revenue	1,899	1,912	3,804	3,763
Commissions, fees and other revenues	1,892	1,914	3,785	3,740
Fiduciary investment income	8	12	20	23
Operating income from continuing operations before income taxes	384	378	750	725
Retail brokerage
Segment Reporting Information
Commissions, fees and other revenues	1,511	1,533	3,005	2,986
Reinsurance brokerage
Segment Reporting Information
Commissions, fees and other revenues	381	381	780	754
HR Solutions
Segment Reporting Information
Revenue	931	906	1,876	1,821
Commissions, fees and other revenues	931	931	1,876	1,821
Operating income from continuing operations before income taxes	58	95	131	178
Consulting services
Segment Reporting Information
Commissions, fees and other revenues	366	366	746	738
Outsourcing
Segment Reporting Information
Commissions, fees and other revenues	570	570	1,138	1,097
Intrasegment
Segment Reporting Information
Commissions, fees and other revenues	(5)	(5)	(8)	(14)
Intersegment elimination
Segment Reporting Information
Revenue	(9)	(7)	(18)	(14)
Commissions, fees and other revenues	(9)	(7)	(18)	(14)
Unallocated Expense
Segment Reporting Information
Operating income from continuing operations before income taxes	$ (48)	$ (33)	$ (85)	$ (65)