Restructuring (Details 2) (USD $) In Millions	9 Months Ended	12 Months Ended
Sep. 30, 2011	Dec. 31, 2010
Restructuring Reserve
Beginning balance	$ 237	$ 263
Assumed Hewitt restructuring liability		43
Expensed	63	168
Cash payments	(131)	(232)
Foreign exchange translation and other	6	(5)
Ending balance	175	237
Aon Hewitt Restructuring Plan
Restructuring Reserve
Beginning balance	88
Assumed Hewitt restructuring liability		43
Expensed	73	52
Cash payments	(70)	(8)
Foreign exchange translation and other	1	1
Ending balance	92	88
Aon Benfield Restructuring Plan
Restructuring Reserve
Beginning balance	26	45
Expensed	(2)	24
Cash payments	(14)	(38)
Foreign exchange translation and other	(1)	(5)
Ending balance	9	26
2007 Restructuring Plan
Restructuring Reserve
Beginning balance	113	202
Expensed	(8)	92
Cash payments	(45)	(178)
Foreign exchange translation and other	6	(3)
Ending balance	66	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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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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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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- 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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- 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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- 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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