Restructuring (Details 2) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Restructuring Reserve
Balance at December 31, 2012	$ 137
Expensed	76
Cash payments	(78)
Foreign exchange translation and other	4
Balance at June 30, 2013	139
Aon Hewitt Plan [Member]
Restructuring Reserve
Balance at December 31, 2012	96
Expensed	76
Cash payments	(58)
Foreign exchange translation and other	0
Balance at June 30, 2013	114
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 June 30, 2013	2
2007 Plan [Member]
Restructuring Reserve
Balance at December 31, 2012	35
Expensed	0
Cash payments	(18)
Foreign exchange translation and other	4
Balance at June 30, 2013	21
Other [Member]
Restructuring Reserve
Balance at December 31, 2012	3
Expensed	0
Cash payments	(1)
Foreign exchange translation and other	0
Balance at June 30, 2013	$ 2