Restructuring (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended	3 Months Ended	9 Months Ended		3 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		3 Months Ended	9 Months Ended	12 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2011 Aon Hewitt Restructuring Plan	Sep. 30, 2011 Aon Hewitt Restructuring Plan	Dec. 31, 2010 Aon Hewitt Restructuring Plan	Sep. 30, 2011 Aon Hewitt Restructuring Plan Real estate lease realization	Sep. 30, 2011 Aon Hewitt Restructuring Plan Workforce reduction job	Sep. 30, 2011 Aon Hewitt Restructuring Plan Workforce reduction job	Dec. 31, 2010 Aon Hewitt Restructuring Plan Workforce reduction	Oct. 14, 2010 Aon Hewitt Restructuring Plan Workforce reduction job	Sep. 30, 2011 Aon Hewitt Restructuring Plan Lease consolidation	Sep. 30, 2011 Aon Hewitt Restructuring Plan Lease consolidation	Dec. 31, 2010 Aon Hewitt Restructuring Plan Lease consolidation	Sep. 30, 2011 Aon Hewitt Restructuring Plan Asset impairments	Sep. 30, 2011 Aon Hewitt Restructuring Plan Asset impairments	Sep. 30, 2011 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Dec. 31, 2011 Aon Benfield Restructuring Plan	Sep. 30, 2011 Aon Benfield Restructuring Plan	Sep. 30, 2010 Aon Benfield Restructuring Plan	Sep. 30, 2011 Aon Benfield Restructuring Plan	Sep. 30, 2010 Aon Benfield Restructuring Plan	Dec. 31, 2010 Aon Benfield Restructuring Plan	Dec. 31, 2009 Aon Benfield Restructuring Plan	Sep. 30, 2011 Aon Benfield Restructuring Plan Workforce reduction job	Sep. 30, 2011 Aon Benfield Restructuring Plan Workforce reduction job	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	Sep. 30, 2011 Aon Benfield Restructuring Plan Lease consolidation	Sep. 30, 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, 2011 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, 2011 Aon Benfield Restructuring Plan Other costs associated with restructuring	Sep. 30, 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	Dec. 31, 2008 Aon Benfield Restructuring Plan Original Estimate year	Sep. 30, 2011 2007 Restructuring Plan	Sep. 30, 2010 2007 Restructuring Plan	Sep. 30, 2011 2007 Restructuring Plan	Sep. 30, 2010 2007 Restructuring Plan
Restructuring and Related Cost
Number of jobs expected to be eliminated under the plan, low end of the range								1,500
Number of jobs expected to be eliminated under the plan, high end of the range								1,800
Number of jobs eliminated to date under the plan					1,030	1,030																715	715
Number of jobs expected to be eliminated under the plan																										875
Restructuring costs recorded in earnings to date																		$ 79
Estimated additional restructuring costs to be recorded in future earnings															28																							81
Period over which the expected cost will be incurred (in years)																																						3
Purchase price allocation																		53					32					20							1			104
Restructuring benefits																		2																					3		8
Reversal of restructuring liability accrued in prior periods																		18
Lease expenses																(2)		4																					2		5
Restructuring and related charges	26	80	52		15	39	49		8	33	3	3	7	1		3	5	(2)	20	26	55	3	14	15	38		(1)	(17)	7	14		2	2	1	1	2	1			3		95
Restructuring charges total to date		132				88				36			7	1				132					99					24			4				5						740
Estimated Total Cost for Restructuring Period		325		145		180				95			47	3				160					125					26			4				5			185
Restructuring charges paid total to date		$ 132