Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 segment	Mar. 31, 2012
Segment Reporting Information
Number of reportable segments	2
Total revenue	$ 2,915	$ 2,841
Total commissions, fees and other revenue	2,908	2,829
Total fiduciary investment income	7	12
Operating income before income tax	410	402
Interest income	1	3
Interest expense	(52)	(59)
Other income	9	0
Income before income taxes	368	346
Total operating segments
Segment Reporting Information
Operating income before income tax	454	439
Risk Solutions
Segment Reporting Information
Total revenue	1,971	1,905
Total commissions, fees and other revenue	1,964	1,893
Total fiduciary investment income	7	12
Operating income before income tax	403	366
Retail brokerage
Segment Reporting Information
Total commissions, fees and other revenue	1,562	1,494
Reinsurance brokerage
Segment Reporting Information
Total commissions, fees and other revenue	402	399
HR Solutions
Segment Reporting Information
Total revenue	954	945
Total commissions, fees and other revenue	954	945
Total fiduciary investment income	0	0
Operating income before income tax	51	73
Consulting services
Segment Reporting Information
Total commissions, fees and other revenue	382	380
Outsourcing
Segment Reporting Information
Total commissions, fees and other revenue	581	568
Intrasegment
Segment Reporting Information
Total commissions, fees and other revenue	(9)	(3)
Intersegment elimination
Segment Reporting Information
Total revenue	(10)	(9)
Total commissions, fees and other revenue	(10)	(9)
Unallocated Expense
Segment Reporting Information
Operating income before income tax	$ (44)	$ (37)