Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 segment	Mar. 31, 2013
Segment Reporting Information
Number of reportable segments	2
Total revenue	$ 2,947	$ 2,915
Total commissions, fees and other revenue	2,941	2,908
Total fiduciary investment income	6	7
Operating income before income tax	469	410
Interest income	2	1
Interest expense	(58)	(52)
Other income	1	9
Income from continuing operations before income taxes	414	368
Total operating segments [Member]
Segment Reporting Information
Operating income before income tax	512	454
Risk Solutions [Member]
Segment Reporting Information
Total revenue	1,994	1,971
Total commissions, fees and other revenue	1,988	1,964
Total fiduciary investment income	6	7
Operating income before income tax	445	403
Retail brokerage
Segment Reporting Information
Total commissions, fees and other revenue	1,579	1,562
Reinsurance brokerage
Segment Reporting Information
Total commissions, fees and other revenue	409	402
HR Solutions [Member]
Segment Reporting Information
Total revenue	965	954
Total commissions, fees and other revenue	965	954
Total fiduciary investment income	0	0
Operating income before income tax	67	51
Consulting services
Segment Reporting Information
Total commissions, fees and other revenue	384	382
Outsourcing
Segment Reporting Information
Total commissions, fees and other revenue	589	581
Intrasegment
Segment Reporting Information
Total commissions, fees and other revenue	(8)	(9)
Intersegment elimination
Segment Reporting Information
Total revenue	(12)	(10)
Total commissions, fees and other revenue	(12)	(10)
Unallocated Expense
Segment Reporting Information
Operating income before income tax	$ (43)	$ (44)