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Acquisitions and Dispositions (Details 2) (HR Solutions)	3 Months Ended
	Mar. 31, 2012 Business
HR Solutions
Dispositions
Number of dispositions	1

Represents the number of entities disposed of by the reporting entity.

No definition available.

Name:	aon_NumberOfDispositions
Namespace Prefix:	aon_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration