Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 item	Sep. 30, 2011
Segment Information
Number of reportable segments			2
Segment Reporting Information
Revenue	$ 2,737	$ 2,723	$ 8,399	$ 8,293
Commissions, fees and other revenues	2,726	2,708	8,368	8,255
Fiduciary investment income	11	15	31	38
Operating income from continuing operations before income taxes	339	328	1,135	1,166
Interest income	1	4	6	14
Interest expense	(57)	(60)	(173)	(186)
Other (expense) income	(9)	20	3	6
Income from continuing operations before income taxes	274	292	971	1,000
Total operating segments
Segment Reporting Information
Operating income from continuing operations before income taxes	389	372	1,270	1,275
Risk Solutions
Segment Reporting Information
Revenue	1,778	1,772	5,582	5,535
Commissions, fees and other revenues	1,767	1,757	5,551	5,497
Fiduciary investment income	11	15	31	38
Operating income from continuing operations before income taxes	309	311	1,059	1,036
Retail brokerage
Segment Reporting Information
Commissions, fees and other revenues	1,390	1,392	4,395	4,378
Reinsurance brokerage
Segment Reporting Information
Commissions, fees and other revenues	377	365	1,156	1,119
HR Solutions
Segment Reporting Information
Revenue	971	957	2,847	2,778
Commissions, fees and other revenues	971	957	2,847	2,778
Operating income from continuing operations before income taxes	80	61	211	239
Consulting services
Segment Reporting Information
Commissions, fees and other revenues	394	403	1,140	1,141
Outsourcing
Segment Reporting Information
Commissions, fees and other revenues	585	558	1,723	1,655
Intrasegment
Segment Reporting Information
Commissions, fees and other revenues	(8)	(4)	(16)	(18)
Intersegment elimination
Segment Reporting Information
Revenue	(12)	(6)	(30)	(20)
Commissions, fees and other revenues	(12)	(6)	(30)	(20)
Unallocated Expense
Segment Reporting Information
Operating income from continuing operations before income taxes	$ (50)	$ (44)	$ (135)	$ (109)