Restructuring Schedule of changes in company's liabilities for its restructuring plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Restructuring Reserve
Beginning balance	$ 137	$ 173	$ 237
Expensed	167	97	106
Cash payments	(152)	(142)	(178)
Stock compensation		(1)
Foreign exchange translation and other	14	10	8
Ending balance	166	137	173
Aon Hewitt Restructuring Plan [Member]
Restructuring Reserve
Beginning balance	96	95	88
Expensed	167	94	98
Cash payments	(125)	(95)	(93)
Stock compensation		0
Foreign exchange translation and other	5	2	2
Ending balance	143	96	95
Aon Benfield Restructuring Plan [Member]
Restructuring Reserve
Beginning balance	3	20	26
Expensed	0	6	19
Cash payments	(2)	(24)	(24)
Stock compensation		(1)
Foreign exchange translation and other	0	2	(1)
Ending balance	1	3	20
2007 Restructuring Plan [Member]
Restructuring Reserve
Beginning balance	35	50	113
Expensed	0	(3)	(11)
Cash payments	(24)	(18)	(59)
Stock compensation		0
Foreign exchange translation and other	9	6	7
Ending balance	20	35	50
Other Restructuring Plan [Member]
Restructuring Reserve
Beginning balance	3	8	10
Expensed	0	0	0
Cash payments	(1)	(5)	(2)
Stock compensation		0
Foreign exchange translation and other	0	0	0
Ending balance	$ 2	$ 3	$ 8