Commitments and Contingencies (Details 2) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012 plan	Dec. 31, 2011
Commitments and Contingencies
Letters of credit outstanding	$ 68	$ 75
Number of US pension plans that are a LOC beneficiary	1

The number of US pension plans that are a beneficiary under letters of credit outstanding as of the reporting date.

No definition available.

Name:	aon_LettersOfCreditPensionPlanBeneficiary
Namespace Prefix:	aon_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant