Restructuring (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	0 Months Ended	3 Months Ended			12 Months Ended			30 Months Ended	3 Months Ended		12 Months Ended			30 Months Ended	3 Months Ended		12 Months Ended			30 Months Ended	0 Months Ended	3 Months Ended		12 Months Ended			30 Months Ended	9 Months Ended	0 Months Ended	3 Months Ended		12 Months Ended			30 Months Ended	3 Months Ended		12 Months Ended			30 Months Ended	3 Months Ended		12 Months Ended						30 Months Ended
	Mar. 31, 2013	Oct. 14, 2010 Aon Hewitt Plan job_elimination	Mar. 31, 2013 Aon Hewitt Plan		Mar. 31, 2012 Aon Hewitt Plan	Dec. 31, 2012 Aon Hewitt Plan	Dec. 31, 2011 Aon Hewitt Plan	Dec. 31, 2010 Aon Hewitt Plan	Mar. 31, 2013 Aon Hewitt Plan	Mar. 31, 2013 Aon Hewitt Plan HR Solutions		Dec. 31, 2012 Aon Hewitt Plan HR Solutions	Dec. 31, 2011 Aon Hewitt Plan HR Solutions	Dec. 31, 2010 Aon Hewitt Plan HR Solutions	Mar. 31, 2013 Aon Hewitt Plan HR Solutions	Mar. 31, 2013 Aon Hewitt Plan Risk Solutions		Dec. 31, 2012 Aon Hewitt Plan Risk Solutions	Dec. 31, 2011 Aon Hewitt Plan Risk Solutions	Dec. 31, 2010 Aon Hewitt Plan Risk Solutions	Mar. 31, 2013 Aon Hewitt Plan Risk Solutions	Oct. 14, 2010 Aon Hewitt Plan Workforce reduction	Mar. 31, 2013 Aon Hewitt Plan Workforce reduction		Dec. 31, 2012 Aon Hewitt Plan Workforce reduction	Dec. 31, 2011 Aon Hewitt Plan Workforce reduction	Dec. 31, 2010 Aon Hewitt Plan Workforce reduction	Mar. 31, 2013 Aon Hewitt Plan Workforce reduction	Sep. 30, 2012 Aon Hewitt Plan Workforce reduction Minimum job_elimination	Oct. 14, 2010 Aon Hewitt Plan Real estate lease rationalization	Mar. 31, 2013 Aon Hewitt Plan Lease consolidation		Dec. 31, 2012 Aon Hewitt Plan Lease consolidation	Dec. 31, 2011 Aon Hewitt Plan Lease consolidation	Dec. 31, 2010 Aon Hewitt Plan Lease consolidation	Mar. 31, 2013 Aon Hewitt Plan Lease consolidation	Mar. 31, 2013 Aon Hewitt Plan Asset impairments		Dec. 31, 2012 Aon Hewitt Plan Asset impairments	Dec. 31, 2011 Aon Hewitt Plan Asset impairments	Dec. 31, 2010 Aon Hewitt Plan Asset impairments	Mar. 31, 2013 Aon Hewitt Plan Asset impairments	Mar. 31, 2013 Aon Hewitt Plan Other costs associated with restructuring		Dec. 31, 2012 Aon Hewitt Plan Other costs associated with restructuring		Dec. 31, 2011 Aon Hewitt Plan Other costs associated with restructuring		Dec. 31, 2010 Aon Hewitt Plan Other costs associated with restructuring		Mar. 31, 2013 Aon Hewitt Plan Other costs associated with restructuring
Restructuring and Related Cost
Number of jobs expected to be eliminated under the plan		2,200
Estimated Total Cost for Restructuring Period		$ 325	$ 325	[1],[2]						$ 213	[1]					$ 112	[1]					$ 213	$ 213	[2]						$ 112	$ 86	[2]					$ 21	[2]					$ 5	[2],[3]
Number of jobs eliminated to date under the plan																													2,180
Restructuring and related charges			26		12				281						182						99							211								55						11									4	[3]
Expensed	26		26			98	105	52		15		66	49	52		11		32	56	0			24		74	64	49				2		18	32	3		0		4	7	0		0	[3]	2	[3]	2	[3]	0	[3]
Restructuring charges reclassified from the HR Solutions segment to the Risk Solutions segment							$ 41

[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.