Restructuring (Details 2) (USD $) In Millions	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Restructuring Reserve
Beginning balance	$ 237	$ 263
Assumed Hewitt restructuring liability		43
Expensed	40	168
Cash payments	(100)	(232)
Foreign exchange translation and other	8	(5)
Ending balance	185	237
Aon Hewitt Restructuring Plan
Restructuring Reserve
Beginning balance	88
Assumed Hewitt restructuring liability		43
Expensed	50	52
Cash payments	(49)	(8)
Foreign exchange translation and other	2	1
Ending balance	91	88
Aon Benfield Restructuring Plan
Restructuring Reserve
Beginning balance	26	45
Expensed	(5)	24
Cash payments	(12)	(38)
Foreign exchange translation and other	1	(5)
Ending balance	10	26
2007 Restructuring Plan
Restructuring Reserve
Beginning balance	113	202
Expensed	(5)	92
Cash payments	(37)	(178)
Foreign exchange translation and other	5	(3)
Ending balance	76	113
Other Restructuring Plan
Restructuring Reserve
Beginning balance	10	16
Cash payments	(2)	(8)
Foreign exchange translation and other		2
Ending balance	$ 8	$ 10

The amount of acquisition cost of a business combination allocated to liabilities assumed.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 141
-Paragraph 37
-Subparagraph g

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Emerging Issues Task Force (EITF)
-Number 98-1

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying amount (including both current and noncurrent portions of the accrual) as of the balance sheet date pertaining to a specified type of cost associated with exit from or disposal of business activities or restructuring pursuant to a duly authorized plan, excluding costs or losses pertaining to an entity newly acquired in a business combination and to asset retirement obligations.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reserve increase representing the amount charged against earnings in the period for a specified incurred and estimated type of cost associated with exit from or disposal of business activities or restructuring pursuant to a duly authorized plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 146

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Data Type:	xbrli:monetaryItemType
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Data Type:	xbrli:stringItemType
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- Definition

Amount of cash paid in the period to fully or partially settle a specified, previously accrued type of restructuring cost.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Emerging Issues Task Force (EITF)
-Number 95-3

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 5
-Section P
-Subsection 3, 4

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 146

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Balance Type:	debit
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- Definition

Amount of foreign currency translation adjustment increasing or decreasing the accrual for a specified type of restructuring cost.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
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