Restructuring (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Restructuring Reserve
Beginning balance	$ 173	$ 237
Expensed	19	105
Cash payments	(35)	(178)
Foreign exchange translation and other	2	9
Ending balance	159	173
Aon Hewitt Restructuring Plan
Restructuring Reserve
Beginning balance	95	88
Expensed	11	98
Cash payments	(19)	(93)
Foreign exchange translation and other	1	2
Ending balance	88	95
Aon Benfield Restructuring Plan
Restructuring Reserve
Beginning balance	20	26
Expensed	8	19
Cash payments	(9)	(24)
Foreign exchange translation and other	2	(1)
Ending balance	21	20
2007 Restructuring Plan
Restructuring Reserve
Beginning balance	50	113
Expensed		(12)
Cash payments	(6)	(59)
Foreign exchange translation and other	(1)	8
Ending balance	43	50
Other Restructuring Plan
Restructuring Reserve
Beginning balance	8	10
Cash payments	(1)	(2)
Ending balance	$ 7	$ 8