Restructuring (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Restructuring Reserve
Balance at December 31, 2012	$ 137
Expensed	26
Cash payments	(42)
Foreign exchange translation and other	3
Balance at March 31, 2013	124
Aon Hewitt Plan
Restructuring Reserve
Balance at December 31, 2012	96
Expensed	26	98	105	52
Cash payments	(28)
Foreign exchange translation and other	1
Balance at March 31, 2013	95	96
Aon Benfield Plan
Restructuring Reserve
Balance at December 31, 2012	3
Expensed	0
Cash payments	(1)
Foreign exchange translation and other	0
Balance at March 31, 2013	2
2007 Plan
Restructuring Reserve
Balance at December 31, 2012	35
Expensed	0
Cash payments	(13)
Foreign exchange translation and other	2
Balance at March 31, 2013	24
Other
Restructuring Reserve
Balance at December 31, 2012	3
Expensed	0
Cash payments	0
Foreign exchange translation and other	0
Balance at March 31, 2013	$ 3