v2.4.0.6

Commitments and Contingencies (Details 2) (USD $) | Dec. 31, 2011 plan | Dec. 31, 2010
--- | --- | ---
Commitments and Contingencies |  |
Letters of credit outstanding | $ 75,000,000 | $ 71,000,000
Number of US pension plans that are a LOC beneficiary | 1 |

X
- Definition The number of US pension plans that are a beneficiary under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | aon_LettersOfCreditPensionPlanBeneficiary
Namespace Prefix: | aon_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant