Segment Information - Schedule of commissions, fees, and other revenue by products (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Commissions, fees and other	$ 3,292	$ 2,873	$ 2,913	$ 2,941	$ 3,202	$ 2,786	$ 2,891	$ 2,908	$ 12,019	$ 11,787	$ 11,476
Revenue	3,299	2,880	2,919	2,947	3,209	2,794	2,897	2,915	12,045	11,815	11,514
Risk Solutions [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									7,808	7,761	7,594
Revenue									7,834	7,789	7,632
Retail brokerage [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									6,334	6,256	6,089
Reinsurance brokerage [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									1,474	1,505	1,505
HR Solutions [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									4,264	4,057	3,925
Revenue									4,264	4,057	3,925
Consulting services [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									1,700	1,626	1,585
Outsourcing [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									2,607	2,469	2,372
Intrasegment [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									(43)	(38)	(32)
Intersegment elimination [Member]
Segment Reporting Information [Line Items]
Revenue									$ (53)	$ (31)	$ (43)