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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Commissions, fees and other	$ 3,202	$ 2,786	$ 2,891	$ 2,908	$ 3,108	$ 2,726	$ 2,813	$ 2,829	$ 11,787	$ 11,476	$ 11,235
Risk Solutions [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									7,761	7,594	7,485
Retail brokerage [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									6,256	6,089	6,022
Reinsurance brokerage [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									1,505	1,505	1,463
HR Solutions [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									4,057	3,925	3,781
Consulting services [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									1,626	1,585	1,546
Outsourcing [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									2,469	2,372	2,258
Intrasegment [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									(38)	(32)	(23)
Intersegment elimination [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									$ (31)	$ (43)	$ (31)