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Restructuring (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013 Aon Hewitt Restructuring Plan [Member]	Mar. 31, 2013 Aon Hewitt Restructuring Plan [Member] HR Solutions [Member]	Mar. 31, 2013 Aon Hewitt Restructuring Plan [Member] Risk Solutions [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 26	$ 15	$ 11
Restructuring reserve	127	166
Cash payments	40
Foreign exchange translation	$ 1