Fair Value and Financial Instruments (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Fair Value \| Money market funds and highly liquid debt securities
Assets:
Money market funds and highly liquid debt securities	$ 2,500	$ 2,618
Fair Value \| Money market funds
Assets:
Money market funds and highly liquid debt securities	2,450	2,591
Fair Value \| Highly liquid debt securities
Assets:
Money market funds and highly liquid debt securities	50	27
Fair Value \| Corporate bonds
Assets:
Other investments	12	12
Fair Value \| Government bonds.
Assets:
Other investments	3	3
Fair Value \| Interest rate contracts
Assets:
Derivatives	15	15
Fair Value \| Foreign exchange contracts
Assets:
Derivatives	137	159
Liabilities:
Derivatives	180	158
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Money market funds and highly liquid debt securities
Assets:
Money market funds and highly liquid debt securities	2,450	2,591
Significant Other Observable Inputs (Level 2) \| Money market funds and highly liquid debt securities
Assets:
Money market funds and highly liquid debt securities	50	27
Significant Other Observable Inputs (Level 2) \| Government bonds.
Assets:
Other investments	3	3
Significant Other Observable Inputs (Level 2) \| Interest rate contracts
Assets:
Derivatives	15	15
Significant Other Observable Inputs (Level 2) \| Foreign exchange contracts
Assets:
Derivatives	137	159
Liabilities:
Derivatives	180	158
Significant Unobservable (Level 3) Inputs \| Corporate bonds
Assets:
Other investments	$ 12	$ 12

Name:	aon_FairValueAssetsMeasuredOnRecurringBasisAbstract
Namespace Prefix:	aon
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

This element represents money market funds and other highly liquid securities, measured at fair value on a recurring basis.

+ Details

Name:	aon_FairValueAssetsMeasuredOnRecurringBasisMoneyMarketFundsAndHighlyLiquidSecurities
Namespace Prefix:	aon
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion related to other investments valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure.

+ Details

Name:	aon_FairValueAssetsMeasuredOnRecurringBasisOtherInvestments
Namespace Prefix:	aon
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	aon_FairValueLiabilitiesMeasuredOnRecurringBasisAbstract
Namespace Prefix:	aon
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FIN39-1

-Paragraph 10A, 10B

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 4, 17

+ Details

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

- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant