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

Name:	aon_BusinessAcquisitionPurchasePriceAllocationIntangibleAssets
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

Name:	us-gaap_BusinessAcquisitionCostOfAcquiredEntityPurchasePrice
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

Name:	us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillAmount
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

Name:	us-gaap_BusinessAcquisitionPurchasePriceAllocationNoncurrentAssetsAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration