v2.3.0.15

Acquisitions and Dispositions (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended
--- | ---
Sep. 30, 2011 entity | Sep. 30, 2010 entity
Business Acquisition |  |
Consideration | $ 100 | $ 87
Intangible assets: |  |
Goodwill | 58 | 29
Other intangible assets | 29 | 52
Intangible assets | $ 87 | $ 81
HR Solutions |  |
Business Acquisition |  |
Number of companies acquired under business combination | 1 |
Risk Solutions |  |
Business Acquisition |  |
Number of companies acquired under business combination | 2 | 13

X
- Definition The amount of acquisition cost of a business combination allocated to an identifiable intangible asset. + Details
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Namespace Prefix: | aon
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Represents the number of companies acquired by the reporting entity. + Details
Name: | aon_NumberOfAcquisitions
Namespace Prefix: | aon
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The total cost of the acquired entity including the cash paid to shareholders of acquired entities, fair value of debt and equity securities issued to shareholders of acquired entities, the fair value of the liabilities assumed, and direct costs of the acquisition. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 51 -Subparagraph d + Details
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Namespace Prefix: | us-gaap
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + Details
Name: | us-gaap_BusinessAcquisitionLineItems
Namespace Prefix: | us-gaap
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of goodwill arising from a business combination, which is the excess of the cost of the acquired entity over the amounts assigned to assets acquired and liabilities assumed. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 52 -Subparagraph c Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 53 -Subparagraph d + Details
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X
- Definition The amount of acquisition cost of a business combination allocated to current assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 51 -Subparagraph e + Details
Name: | us-gaap_BusinessAcquisitionPurchasePriceAllocationNoncurrentAssets
Namespace Prefix: | us-gaap
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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