Restructuring (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended								1 Months Ended	12 Months Ended
	Dec. 31, 2012 Aon Hewitt Restructuring Plan	Dec. 31, 2011 Aon Hewitt Restructuring Plan	Dec. 31, 2010 Aon Hewitt Restructuring Plan	Dec. 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, 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	Dec. 31, 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	Dec. 31, 2012 Aon Hewitt Restructuring Plan Workforce reduction Minimum item	Dec. 31, 2012 Aon Hewitt Restructuring Plan Real estate lease rationalization	Dec. 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	Dec. 31, 2012 Aon Hewitt Restructuring Plan Asset impairments	Dec. 31, 2011 Aon Hewitt Restructuring Plan Asset impairments	Dec. 31, 2012 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Dec. 31, 2011 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Dec. 31, 2012 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	Dec. 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 item	Dec. 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	Dec. 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	Dec. 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									2,000																					810
Estimated Total Cost for Restructuring Period	$ 325			$ 226			$ 99			$ 192				$ 133	$ 95			$ 33		$ 5
Number of jobs eliminated to date under the plan													1,900
Restructuring and related charges	98	105	52	66	49	52	32	56		74	64	49			18	32	3	4	7	2	2	6	19	26	55	6	33	15	38			(15)	7	14		2	2		1	2	1
Restructuring charges reclassified from the HR Solutions segment to the Risk Solutions segment		41
Purchase price allocation																						53				32					20							1
Restructuring charges Total Inception to Date	$ 255			$ 167			$ 88			$ 187					$ 53			$ 11		$ 4		$ 159				$ 124					$ 26				$ 4			$ 5