Restructuring (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended		3 Months Ended	12 Months Ended			3 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended		1 Months Ended		3 Months Ended		12 Months Ended		3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended		12 Months Ended			3 Months Ended	12 Months Ended				3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended
	Mar. 31, 2012 Aon Hewitt Restructuring Plan	Mar. 31, 2011 Aon Hewitt Restructuring Plan	Dec. 31, 2011 Aon Hewitt Restructuring Plan	Dec. 31, 2010 Aon Hewitt Restructuring Plan	Mar. 31, 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	Dec. 31, 2011 Aon Hewitt Restructuring Plan Consulting services	Mar. 31, 2012 Aon Hewitt Restructuring Plan Risk Solutions	Mar. 31, 2011 Aon Hewitt Restructuring Plan Risk Solutions	Dec. 31, 2011 Aon Hewitt Restructuring Plan Risk Solutions	Mar. 31, 2012 Aon Hewitt Restructuring Plan Workforce reduction job	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 job	Oct. 31, 2010 Aon Hewitt Restructuring Plan Workforce reduction Maximum job	Mar. 31, 2012 Aon Hewitt Restructuring Plan Real estate lease realization	Mar. 31, 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	Mar. 31, 2012 Aon Hewitt Restructuring Plan Asset impairments	Dec. 31, 2011 Aon Hewitt Restructuring Plan Asset impairments	Mar. 31, 2012 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Dec. 31, 2011 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Mar. 31, 2012 Aon Benfield Restructuring Plan	Mar. 31, 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	Mar. 31, 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 job	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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				$ 257				$ 68			$ 180					$ 145	$ 95			$ 47		$ 3		$ 161					$ 126					$ 26				$ 4			$ 5
Number of jobs eliminated to date under the plan												1,186
Restructuring and related charges	12	23	105	52	9	49	52	41	3	14	56	7	64	49				4	32	3	1	7		2	8	7	19	26	55	8	33	15	38			(15)	7	14		2	2		1	2	1
Purchase price allocation																									53					32					20							1
Restructuring charges Total Inception to Date	$ 169				$ 110				$ 59			$ 120						$ 39			$ 8		$ 2		$ 161					$ 126					$ 26				$ 4			$ 5