Restructuring (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013 Aon Hewitt Restructuring Plan [Member]	Sep. 30, 2013 Aon Hewitt Restructuring Plan [Member]	Sep. 30, 2013 Aon Hewitt Restructuring Plan [Member] HR Solutions [Member]	Sep. 30, 2013 Aon Hewitt Restructuring Plan [Member] HR Solutions [Member]	Sep. 30, 2013 Aon Hewitt Restructuring Plan [Member] Risk Solutions [Member]	Sep. 30, 2013 Aon Hewitt Restructuring Plan [Member] Risk Solutions [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 30	$ 109	$ 6	$ 57	$ 24	$ 52
Restructuring reserve	90	166
Cash payments	70
Restructuring reserve reduction due to foreign currency translation	$ 6