Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010
Segment Information
Number of reportable segments									2
Segment Reporting Information
Revenue	$ 3,115	$ 2,737	$ 2,821	$ 2,841	$ 2,994	$ 2,723	$ 2,811	$ 2,759	$ 11,514	$ 11,287	$ 8,512
Commissions, fees and other revenues	3,108	2,726	2,813	2,829	2,980	2,708	2,799	2,748	11,476	11,235	8,457
Fiduciary investment income	7	11	8	12	14	15	12	11	38	52	55
Operating income from continuing operations before income taxes	461	339	394	402	430	328	440	398	1,596	1,596	1,244
Interest income									10	18	15
Interest expense									(228)	(245)	(182)
Other income (expense)									3	15	(18)
Income from continuing operations before income taxes									1,381	1,384	1,059
Long-lived assets	13,819				13,789				13,819	13,789
United States.
Segment Reporting Information
Revenue									5,336	5,134	3,400
Long-lived assets	8,355				8,617				8,355	8,617
Americas other than U.S.
Segment Reporting Information
Revenue									1,190	1,176	978
Long-lived assets	599				574				599	574
United Kingdom.
Segment Reporting Information
Revenue									1,541	1,519	1,322
Long-lived assets	1,721				1,589				1,721	1,589
Europe, Middle East and Africa
Segment Reporting Information
Revenue									2,271	2,377	2,035
Long-lived assets	2,462				2,448				2,462	2,448
Asia Pacific
Segment Reporting Information
Revenue									1,176	1,081	777
Long-lived assets	682				561				682	561
Total operating segments
Segment Reporting Information
Revenue									11,514	11,287	8,512
Operating income from continuing operations before income taxes									1,782	1,749	1,449
Risk Solutions
Segment Reporting Information
Revenue									7,632	7,537	6,989
Commissions, fees and other revenues									7,594	7,485	6,935
Fiduciary investment income									38	52	54
Operating income from continuing operations before income taxes									1,493	1,413	1,328
Retail brokerage
Segment Reporting Information
Commissions, fees and other revenues									6,089	6,022	5,491
Reinsurance brokerage
Segment Reporting Information
Commissions, fees and other revenues									1,505	1,463	1,444
HR Solutions
Segment Reporting Information
Revenue									3,925	3,781	1,545
Commissions, fees and other revenues									3,925	3,781	1,544
Fiduciary investment income											1
Operating income from continuing operations before income taxes									289	336	121
Consulting services
Segment Reporting Information
Commissions, fees and other revenues									1,585	1,546	821
Outsourcing
Segment Reporting Information
Commissions, fees and other revenues									2,372	2,258	731
Intrasegment
Segment Reporting Information
Commissions, fees and other revenues									(32)	(23)	(8)
Intersegment elimination
Segment Reporting Information
Revenue									(43)	(31)	(22)
Commissions, fees and other revenues									(43)	(31)	(22)
Unallocated Expense
Segment Reporting Information
Operating income from continuing operations before income taxes									$ (186)	$ (153)	$ (205)