This element represents the length of time it will take to finalize the acquisition method of accounting for a specific acquisition.

Name:	aon_AllocationOfAcquisitionPurchasePriceTime
Namespace Prefix:	aon
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents acquisition, integration-related costs and financing costs incurred that have been recorded in other general expenses.

+ Details

Name:	aon_BusinessCombinationAcquisitionIntegrationAndFinancingCostsIncludedInOtherGeneralExpenses
Namespace Prefix:	aon
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents the portion of aggregate merger consideration to be paid in cash and total number of shares of the entity as a condition in the Stock Electing Consideration.

+ Details

Name:	aon_PortionOfAggregateMergerConsiderationToBePaidInCashAsPerMergerAgreementPercentage
Namespace Prefix:	aon
Data Type:	us-types:percentItemType
Balance Type:	na
Period Type:	instant

- 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

- 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

Acquisitions and Dispositions (Details 2) (USD $) In Millions, unless otherwise specified			6 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 Hewitt Associates, Inc (Hewitt)	Oct. 01, 2010 Hewitt Associates, Inc (Hewitt)	Jun. 30, 2011 Hewitt Associates, Inc (Hewitt) HR Solutions	Jun. 30, 2011 Hewitt Associates, Inc (Hewitt) HR Solutions
Business Acquisition
Value of cash and stock consideration	$ 92	$ 104		$ 4,932
Percentage of aggregate consideration to be paid in cash and in number of shares each (as a percent)				50.00%
Acquisition, integration and financing cost recorded in Other General Expenses					$ 5	$ 20
Maximum number of years to finalize purchase price allocation (in years)			1