Restructuring Schedule of restructuring and related expenses by type (Details) (Aon Hewitt Restructuring Plan [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	$ 429
Restructuring and related charges	174		98		105		52
Restructuring charges reclassified from the HR Solutions segment to the Risk Solutions segment					41
HR Solutions [Member]
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	247
Restructuring and related charges	80		66		49		52
Risk Solutions [Member]
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	182
Restructuring and related charges	94		32		56		0
Workforce reduction [Member]
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	266
Restructuring and related charges	79		74		64		49
Lease consolidation [Member]
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	136
Restructuring and related charges	83		18		32		3
Asset impairments [Member]
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	18
Restructuring and related charges	7		4		7		0
Other costs associated with restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	9	[1]
Restructuring and related charges	$ 5	[1]	$ 2	[1]	$ 2	[1]	$ 0	[1]

[1] Other costs associated with restructuring initiatives, including moving costs and consulting and legal fees, are recognized when incurred.