Restructuring (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Restructuring Reserve
Beginning balance	$ 173	$ 237	$ 263
Assumed Hewitt restructuring liability			43
Expensed	97	106	168
Cash payments	(142)	(178)	(232)
Stock compensation	(1)
Foreign exchange translation and other	10	8	(5)
Ending balance	137	173	237
Aon Hewitt Restructuring Plan
Restructuring Reserve
Beginning balance	95	88
Assumed Hewitt restructuring liability			43
Expensed	94	98	52
Cash payments	(95)	(93)	(8)
Foreign exchange translation and other	2	2	1
Ending balance	96	95	88
Aon Benfield Restructuring Plan
Restructuring Reserve
Beginning balance	20	26	45
Expensed	6	19	24
Cash payments	(24)	(24)	(38)
Stock compensation	(1)
Foreign exchange translation and other	2	(1)	(5)
Ending balance	3	20	26
2007 Restructuring Plan
Restructuring Reserve
Beginning balance	50	113	202
Expensed	(3)	(11)	92
Cash payments	(18)	(59)	(178)
Foreign exchange translation and other	6	7	(3)
Ending balance	35	50	113
Other Restructuring Plan
Restructuring Reserve
Beginning balance	8	10	16
Cash payments	(5)	(2)	(8)
Foreign exchange translation and other			2
Ending balance	$ 3	$ 8	$ 10