Restructuring (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended		9 Months Ended	12 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended	3 Months Ended	9 Months Ended		12 Months Ended			9 Months Ended	12 Months Ended				9 Months Ended	12 Months Ended			9 Months Ended	12 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2012 Aon Hewitt Restructuring Plan	Sep. 30, 2011 Aon Hewitt Restructuring Plan	Sep. 30, 2012 Aon Hewitt Restructuring Plan	Sep. 30, 2011 Aon Hewitt Restructuring Plan	Dec. 31, 2011 Aon Hewitt Restructuring Plan	Dec. 31, 2010 Aon Hewitt Restructuring Plan	Sep. 30, 2012 Aon Hewitt Restructuring Plan HR Solutions	Sep. 30, 2012 Aon Hewitt Restructuring Plan HR Solutions	Dec. 31, 2011 Aon Hewitt Restructuring Plan HR Solutions	Dec. 31, 2010 Aon Hewitt Restructuring Plan HR Solutions	Sep. 30, 2011 Aon Hewitt Restructuring Plan Consulting services	Dec. 31, 2011 Aon Hewitt Restructuring Plan Consulting services	Sep. 30, 2012 Aon Hewitt Restructuring Plan Risk Solutions	Sep. 30, 2012 Aon Hewitt Restructuring Plan Risk Solutions	Dec. 31, 2011 Aon Hewitt Restructuring Plan Risk Solutions	Oct. 31, 2010 Aon Hewitt Restructuring Plan Workforce reduction item	Sep. 30, 2012 Aon Hewitt Restructuring Plan Workforce reduction	Sep. 30, 2012 Aon Hewitt Restructuring Plan Workforce reduction	Dec. 31, 2011 Aon Hewitt Restructuring Plan Workforce reduction	Dec. 31, 2010 Aon Hewitt Restructuring Plan Workforce reduction	Sep. 30, 2012 Aon Hewitt Restructuring Plan Workforce reduction Minimum item	Sep. 30, 2012 Aon Hewitt Restructuring Plan Real estate lease rationalization	Sep. 30, 2012 Aon Hewitt Restructuring Plan Lease consolidation	Sep. 30, 2012 Aon Hewitt Restructuring Plan Lease consolidation	Dec. 31, 2011 Aon Hewitt Restructuring Plan Lease consolidation	Dec. 31, 2010 Aon Hewitt Restructuring Plan Lease consolidation	Sep. 30, 2012 Aon Hewitt Restructuring Plan Asset impairments	Dec. 31, 2011 Aon Hewitt Restructuring Plan Asset impairments	Sep. 30, 2012 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Sep. 30, 2012 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Dec. 31, 2011 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Sep. 30, 2011 Aon Benfield Restructuring Plan	Sep. 30, 2012 Aon Benfield Restructuring Plan	Sep. 30, 2011 Aon Benfield Restructuring Plan	Dec. 31, 2011 Aon Benfield Restructuring Plan	Dec. 31, 2010 Aon Benfield Restructuring Plan	Dec. 31, 2009 Aon Benfield Restructuring Plan	Sep. 30, 2012 Aon Benfield Restructuring Plan Workforce reduction	Dec. 31, 2011 Aon Benfield Restructuring Plan Workforce reduction	Dec. 31, 2010 Aon Benfield Restructuring Plan Workforce reduction	Dec. 31, 2009 Aon Benfield Restructuring Plan Workforce reduction	Dec. 31, 2008 Aon Benfield Restructuring Plan Workforce reduction item	Sep. 30, 2012 Aon Benfield Restructuring Plan Lease consolidation	Dec. 31, 2011 Aon Benfield Restructuring Plan Lease consolidation	Dec. 31, 2010 Aon Benfield Restructuring Plan Lease consolidation	Dec. 31, 2009 Aon Benfield Restructuring Plan Lease consolidation	Sep. 30, 2012 Aon Benfield Restructuring Plan Asset impairments	Dec. 31, 2010 Aon Benfield Restructuring Plan Asset impairments	Dec. 31, 2009 Aon Benfield Restructuring Plan Asset impairments	Sep. 30, 2012 Aon Benfield Restructuring Plan Other costs associated with restructuring	Dec. 31, 2011 Aon Benfield Restructuring Plan Other costs associated with restructuring	Dec. 31, 2010 Aon Benfield Restructuring Plan Other costs associated with restructuring	Dec. 31, 2009 Aon Benfield Restructuring Plan Other costs associated with restructuring
Restructuring and Related Cost
Number of jobs expected to be eliminated under the plan																2,000																										810
Estimated Total Cost for Restructuring Period			$ 325					$ 243						$ 82				$ 180				$ 145		$ 95			$ 47			$ 3			$ 161					$ 126					$ 26				$ 4			$ 5
Number of jobs eliminated to date under the plan																					1,800
Restructuring and related charges	32	26	57	80	105	52	16	36	49	52	14	41	16	21	56		25	44	64	49			6	11	32	3	1	7	1	1	2	3	8		19	26	55	8	33	15	38			(15)	7	14		2	2	1	1	2	1
Restructuring charges Total Inception to Date			214					137						77				157						46			8			3			161					126					26				4			5
Restructuring benefits																																		2
Purchase price allocation																																	$ 53					$ 32					$ 20							$ 1