Restructuring (Details 2) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Restructuring Reserve
Beginning balance	$ 173
Expensed	32
Cash payments	(70)
Foreign exchange translation and other	(1)
Ending balance	134
Aon Hewitt Restructuring Plan
Restructuring Reserve
Beginning balance	95
Expensed	24
Cash payments	(39)
Ending balance	80
Aon Benfield Restructuring Plan
Restructuring Reserve
Beginning balance	20
Expensed	8
Cash payments	(19)
Foreign exchange translation and other	1
Ending balance	10
2007 Restructuring Plan
Restructuring Reserve
Beginning balance	50
Cash payments	(10)
Foreign exchange translation and other	(2)
Ending balance	38
Other Restructuring Plan
Restructuring Reserve
Beginning balance	8
Cash payments	(2)
Ending balance	$ 6