Acquisitions and Dispositions (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013 Risk Solutions business	Mar. 31, 2012 HR Solutions business
Dispositions
Number of dispositions	3	1
Gain (loss) recognized on sale	$ 0	$ 0

The gain (loss) resulting from the sale of a disposal group that is not a discontinued operation and other income. It is included in income from continuing operations before income taxes in the income statement.

No definition available.

Name:	aon_DisposalGroupNotDiscontinuedOperationGainLossOnDisposalAndOther
Namespace Prefix:	aon_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration