v2.3.0.15

Stock Compensation Plans (Details) (USD $) In Millions, except Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Stock Compensation Plans
Restricted stock units ("RSUs")	$ 31	$ 29	$ 108	$ 99
Performance plans	24	10	59	51
Stock options	2	3	7	13
Employee stock purchase plans	2	1	5	3
Total stock compensation expense	59	43	179	166
Status of non-vested stock awards
Non-vested at beginning of period (in shares)			10,674,000	12,850,000
Granted (in shares)			4,831,000	4,984,000
Vested (in shares)			(4,850,000)	(6,288,000)
Forfeited (in shares)			(410,000)	(412,000)
Non-vested at end of period (in shares)	10,245,000	11,134,000	10,245,000	11,134,000
Weighted Average Fair value
Non-vested at beginning of period (in dollars per share)			$ 38	$ 36
Granted (in dollars per share)			$ 50	$ 39
Vested (in dollars per share)			$ 42	$ 35
Forfeited (in dollars per share)			$ 39	$ 38
Non-vested at end of period (in dollars per share)	$ 42	$ 38	$ 42	$ 38
Potential RSUs to be issued based on current performance levels (in shares)	5,791,000	4,883,000	5,791,000	4,883,000
Unamortized expense, based on current performance levels	$ 107	$ 107	$ 107	$ 107
2008 Leadership Performance Plan ("LPP") cycle
Employee Stock Options and Stock Awards
Shares granted			1,200,000
2007 Leadership Performance Plan ("LPP") cycle
Employee Stock Options and Stock Awards
Shares granted				1,700,000
2006 performance plan
Employee Stock Options and Stock Awards
Shares granted			300,000
Incentive compensation plans
Employee Stock Options and Stock Awards
Shares granted			3,300,000	3,300,000

X

- Definition

This element represents the expense associated with the entity's employee stock purchase plans.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The expense associated with the entity's performance based restricted stock awards.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

This element represents number of shares of potential restricted stock units to be issued for performance-based stock award programs, based on current performance levels.

+ Details

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- Details

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X

- Definition

The weighted average fair value as of the grant date of share-based award plans other than stock option plans that were not exercised or put into effect during the reporting period as a result of the occurrence of a terminating event specified in the contractual agreement of the plan.

+ Details

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X

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X

- Details

Name:	aon_StockCompensationPlansDisclosureAbstract
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Data Type:	xbrli:stringItemType
Balance Type:	na
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X

- Definition

Represents unamortized expense of performance-based stock award plans, based on current performance levels.

+ Details

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

X

- Definition

Represents the expense recognized during the period arising from share-based compensation arrangements (for example, shares of stock, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph 64

-Subparagraph b



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph g(1)



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section F

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The noncash expense that represents the cost of restricted stock distributed to employees as compensation.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

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- Details

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X

- Definition

The number of shares under a share-based award plan other than a stock option plan that were settled during the reporting period due to a failure to satisfy vesting conditions pertaining to all option plans.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(e)

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Data Type:	xbrli:sharesItemType
Balance Type:	na
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X

- Definition

The number of shares issuable under a share-based award plan pertaining to grants made during the period on other than stock option plans (for example, phantom stock plan, stock appreciation rights plan, performance target plan).

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(c)

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Data Type:	xbrli:sharesItemType
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X

- Definition

The weighted average fair value at grant for nonvested share-based awards issued during the period on other than stock option plans (for example, phantom stock plan, stock appreciation rights plan, performance target plan).

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(c)



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph c(1)

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X

- Definition

The number of outstanding awards on nonstock option plans (for example, phantom stock plan, stock appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(a)



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(b)

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:sharesItemType
Balance Type:	na
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X

- Definition

The weighted average fair value of nonvested awards on share-based plans excluding option plans (for example, phantom stock plan, stock appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(a)



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(b)

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Data Type:	us-types:perShareItemType
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X

- Definition

The decrease in the number of shares potentially issuable under a share-based award plan pertaining to awards for which the grantee has gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares, other instruments, or cash in accordance with the terms of the arrangement.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(d)

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Namespace Prefix:	us-gaap
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The weighted average fair value as of grant dates pertaining to a share-based award plan other than a stock option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares, other instruments, or cash in accordance with the terms of the arrangement.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(d)

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Namespace Prefix:	us-gaap
Data Type:	us-types:perShareItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of stock granted during the period as a result of any share-based compensation plan other than en employee stock ownership plan

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph 43

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Data Type:	xbrli:sharesItemType
Balance Type:	na
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X

- Definition

The noncash expense that accounts for the value of stock options distributed to employees as compensation.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph 64



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph i

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Balance Type:	debit
Period Type:	duration