Restructuring (Details) (Aon Hewitt Plan [Member], USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended			9 Months Ended			12 Months Ended						36 Months Ended
	Oct. 14, 2010	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Sep. 30, 2013 job_elimination
Restructuring and Related Cost
Number of jobs eliminated to date under the plan														2,660
Estimated Total Cost for Restructuring Period	$ 411				$ 411	[1],[2]
Restructuring and related charges		30		32	109		57							364	[1]
Expensed		30	[1]		109	[1]		98	[1]	105	[1]	52	[1]
Restructuring charges reclassified from the HR Solutions segment to the Risk Solutions segment										41
HR Solutions [Member]
Restructuring and Related Cost
Estimated Total Cost for Restructuring Period					244	[1]
Restructuring and related charges														224
Expensed		6			57			66		49		52
Risk Solutions [Member]
Restructuring and Related Cost
Estimated Total Cost for Restructuring Period					167	[1]
Restructuring and related charges														140
Expensed		24			52			32		56		0
Workforce reduction [Member]
Restructuring and Related Cost
Estimated Total Cost for Restructuring Period	261				261	[2]
Restructuring and related charges														237
Expensed		12			50			74		64		49
Workforce reduction [Member] | Minimum [Member]
Restructuring and Related Cost
Number of jobs eliminated to date under the plan	2,900
Real estate lease rationalization [Member]
Restructuring and Related Cost
Estimated Total Cost for Restructuring Period	150
Lease consolidation [Member]
Restructuring and Related Cost
Estimated Total Cost for Restructuring Period					128	[2]
Restructuring and related charges														106
Expensed		17			53			18		32		3
Asset impairments [Member]
Restructuring and Related Cost
Estimated Total Cost for Restructuring Period					15	[2]
Restructuring and related charges														14
Expensed		0			3			4		7		0
Other costs associated with restructuring [Member]
Restructuring and Related Cost
Estimated Total Cost for Restructuring Period					7	[2],[3]
Restructuring and related charges														7	[3]
Expensed		$ 1	[3]		$ 3	[3]		$ 2	[3]	$ 2	[3]	$ 0	[3]

[1] Costs included in the Risk Solutions segment are associated with the transfer of the health and benefits consulting business from HR Solutions to Risk Solutions effective January 1, 2012. Costs incurred in 2011 in the HR Solutions segment of $41 million related to the health and benefits consulting business have been reclassified and presented in the Risk Solutions segment.

[2] Actual costs, when incurred, will vary due to changes in the assumptions built into this plan. Significant assumptions that may change when plans are finalized and implemented include, but are not limited to, changes in severance calculations, changes in the assumptions underlying sublease loss calculations due to changing market conditions, and changes in the overall analysis that might cause the Company to add or cancel component initiatives.

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