Restructuring (Details 2) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Restructuring Reserve
Balance at December 31, 2012	$ 137
Expensed	106
Cash payments	(113)
Foreign exchange translation and other	13
Balance at September 30, 2013 (1)	143	[1]
Aon Hewitt Plan [Member]
Restructuring Reserve
Balance at December 31, 2012	96
Expensed	106
Cash payments	(91)
Foreign exchange translation and other	7
Balance at September 30, 2013 (1)	118	[1]
Aon Benfield Plan [Member]
Restructuring Reserve
Balance at December 31, 2012	3
Expensed	0
Cash payments	(1)
Foreign exchange translation and other	0
Balance at September 30, 2013 (1)	2	[1]
2007 Plan [Member]
Restructuring Reserve
Balance at December 31, 2012	35
Expensed	0
Cash payments	(20)
Foreign exchange translation and other	6
Balance at September 30, 2013 (1)	21	[1]
Other [Member]
Restructuring Reserve
Balance at December 31, 2012	3
Expensed	0
Cash payments	(1)
Foreign exchange translation and other	0
Balance at September 30, 2013 (1)	$ 2	[1]

[1] Included within Accounts payable and accrued liabilities and Other non-current liabilities