Restructuring (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended		3 Months Ended	6 Months Ended	12 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended		3 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				3 Months Ended	6 Months Ended
	Dec. 31, 2010	Jun. 30, 2011 Aon Hewitt Restructuring Plan	Jun. 30, 2011 Aon Hewitt Restructuring Plan	Dec. 31, 2010 Aon Hewitt Restructuring Plan	Jun. 30, 2011 Aon Hewitt Restructuring Plan Real estate lease rationalization	Jun. 30, 2011 Aon Hewitt Restructuring Plan Workforce reduction	Jun. 30, 2011 Aon Hewitt Restructuring Plan Workforce reduction	Dec. 31, 2010 Aon Hewitt Restructuring Plan Workforce reduction	Oct. 14, 2010 Aon Hewitt Restructuring Plan Workforce reduction	Jun. 30, 2011 Aon Hewitt Restructuring Plan Lease consolidation	Jun. 30, 2011 Aon Hewitt Restructuring Plan Lease consolidation	Dec. 31, 2010 Aon Hewitt Restructuring Plan Lease consolidation	Jun. 30, 2011 Aon Hewitt Restructuring Plan Asset impairments	Jun. 30, 2011 Aon Hewitt Restructuring Plan Asset impairments	Jun. 30, 2011 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Jun. 30, 2011 Aon Hewitt Restructuring Plan Other costs associated with restructuring	Jun. 30, 2011 Aon Benfield Restructuring Plan	Jun. 30, 2010 Aon Benfield Restructuring Plan	Jun. 30, 2011 Aon Benfield Restructuring Plan	Jun. 30, 2010 Aon Benfield Restructuring Plan	Dec. 31, 2010 Aon Benfield Restructuring Plan	Dec. 31, 2009 Aon Benfield Restructuring Plan	Jun. 30, 2011 Aon Benfield Restructuring Plan Workforce reduction	Jun. 30, 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	Jun. 30, 2011 Aon Benfield Restructuring Plan Lease consolidation	Jun. 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	Jun. 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	Jun. 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	Dec. 31, 2009 Aon Benfield Restructuring Plan Adjustment	Jun. 30, 2010 2007 Restructuring Plan	Jun. 30, 2011 2007 Restructuring Plan	Jun. 30, 2010 2007 Restructuring Plan
Restructuring and Related Cost
Number of employees expected to be eliminated under the plan, low end of the range									1,500
Number of employees expected to be eliminated under the plan, high end of the range									1,800
Number of employees eliminated to date under the plan						860	860																705	705
Number of employees expected to be eliminated under the plan																											730
Restructuring costs recorded in earnings to date																			$ 76
Estimated additional restructuring costs to be recorded in future earnings																			7																			81
Period over which the expected cost will be incurred (in years)																																						3
Purchase price allocation	43			43																				32					22						1			104	(49)
Restructuring benefits																	12		5
Reversal of restructuring liability accrued in prior periods																	17																								5
Lease expenses																			6																						3
Restructuring and related charges		31	54	52		6	24	49		20	25	3	4	4	1	1	(12)	6	(5)	15	26	55	4	11	15	38		(16)	(16)	7	14		2	2		2	1			25		92
Restructuring charges total to date			106				73				28			4		1			131					96					27			4			4						743
Estimated Total Cost for Restructuring Period			325		145		180				95			47		3			138					101					29			4			4			185
Restructuring charges paid total to date			$ 116