Restructuring (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended		3 Months Ended	6 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended		1 Months Ended		6 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended		6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	3 Months Ended	6 Months Ended		12 Months Ended			6 Months Ended	12 Months Ended				6 Months Ended	12 Months Ended			6 Months Ended	12 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2012 Aon Hewitt Restructuring Plan	Jun. 30, 2011 Aon Hewitt Restructuring Plan	Jun. 30, 2012 Aon Hewitt Restructuring Plan	Jun. 30, 2011 Aon Hewitt Restructuring Plan	Dec. 31, 2011 Aon Hewitt Restructuring Plan	Dec. 31, 2010 Aon Hewitt Restructuring Plan	Jun. 30, 2012 Aon Hewitt Restructuring Plan HR Solutions	Jun. 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	Jun. 30, 2011 Aon Hewitt Restructuring Plan Consulting services	Dec. 31, 2011 Aon Hewitt Restructuring Plan Consulting services	Jun. 30, 2012 Aon Hewitt Restructuring Plan Risk Solutions	Jun. 30, 2012 Aon Hewitt Restructuring Plan Risk Solutions	Dec. 31, 2011 Aon Hewitt Restructuring Plan Risk Solutions	Jun. 30, 2012 Aon Hewitt Restructuring Plan Workforce reduction	Jun. 30, 2012 Aon Hewitt Restructuring Plan Workforce reduction item	Dec. 31, 2011 Aon Hewitt Restructuring Plan Workforce reduction	Dec. 31, 2010 Aon Hewitt Restructuring Plan Workforce reduction	Oct. 31, 2010 Aon Hewitt Restructuring Plan Workforce reduction Minimum item	Oct. 31, 2010 Aon Hewitt Restructuring Plan Workforce reduction Maximum item	Jun. 30, 2012 Aon Hewitt Restructuring Plan Real estate lease realization	Jun. 30, 2012 Aon Hewitt Restructuring Plan Lease consolidation	Jun. 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	Jun. 30, 2012 Aon Hewitt Restructuring Plan Asset impairments	Dec. 31, 2011 Aon Hewitt Restructuring Plan Asset impairments	Jun. 30, 2012 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Dec. 31, 2011 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Jun. 30, 2011 Aon Benfield Restructuring Plan	Jun. 30, 2012 Aon Benfield Restructuring Plan	Jun. 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	Jun. 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	Jun. 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	Jun. 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	Jun. 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																				1,500	1,800																				810
Estimated Total Cost for Restructuring Period			$ 325					$ 255						$ 70			$ 180					$ 145		$ 95			$ 47		$ 3			$ 161					$ 126					$ 26				$ 4			$ 5
Number of jobs eliminated to date under the plan																	1,540
Restructuring and related charges	13	31	25	54	105	52	11	20	49	52	10	41	2	5	56	12	19	64	49				1	5	32	3	1	7		2		8		19	26	55	8	33	15	38			(15)	7	14		2	2		1	2	1
Restructuring benefits																															12		5
Purchase price allocation																																53					32					20							1
Restructuring charges Total Inception to Date			$ 182					$ 121						$ 61			$ 132							$ 40			$ 8		$ 2			$ 161					$ 126					$ 26				$ 4			$ 5