Restructuring (Details 2) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Restructuring Reserve
Beginning balance	$ 173
Expensed	64
Cash payments	(109)
Foreign exchange translation and other	3
Ending balance	131
Aon Hewitt Restructuring Plan
Restructuring Reserve
Beginning balance	95
Expensed	56
Cash payments	(71)
Ending balance	80
Aon Benfield Restructuring Plan
Restructuring Reserve
Beginning balance	20
Expensed	8
Cash payments	(20)
Foreign exchange translation and other	1
Ending balance	9
2007 Restructuring Plan
Restructuring Reserve
Beginning balance	50
Cash payments	(15)
Foreign exchange translation and other	2
Ending balance	37
Other Restructuring Plan
Restructuring Reserve
Beginning balance	8
Cash payments	(3)
Ending balance	$ 5