Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 segment	Mar. 31, 2011
Segment Information
Number of reportable segments	2
Segment Reporting Information
Revenue	$ 2,841	$ 2,759
Commissions, fees and other revenues	2,829	2,748
Fiduciary investment income	12	11
Operating income from continuing operations before income taxes	402	398
Interest income	3	6
Interest expense	(59)	(63)
Other income		15
Income from continuing operations before income taxes	346	356
Total operating segments
Segment Reporting Information
Revenue	2,841	2,759
Operating income from continuing operations before income taxes	439	430
Risk Solutions
Segment Reporting Information
Revenue	1,905	1,851
Commissions, fees and other revenues	1,893	1,840
Fiduciary investment income	12	11
Operating income from continuing operations before income taxes	366	347
Retail brokerage
Segment Reporting Information
Commissions, fees and other revenues	1,494	1,453
Reinsurance brokerage
Segment Reporting Information
Commissions, fees and other revenues	399	387
HR Solutions
Segment Reporting Information
Revenue	945	915
Commissions, fees and other revenues	945	915
Operating income from continuing operations before income taxes	73	83
Consulting services
Segment Reporting Information
Commissions, fees and other revenues	380	371
Outsourcing
Segment Reporting Information
Commissions, fees and other revenues	568	552
Intrasegment
Segment Reporting Information
Commissions, fees and other revenues	(3)	(8)
Intersegment elimination
Segment Reporting Information
Revenue	(9)	(7)
Commissions, fees and other revenues	(9)	(7)
Unallocated Expense
Segment Reporting Information
Operating income from continuing operations before income taxes	$ (37)	$ (32)