Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Segment Information
Number of reportable segments			2
Segment Reporting Information
Revenue	$ 2,811	$ 1,898	$ 5,570	$ 3,802
Commissions, fees and other revenues	2,799	1,883	5,547	3,774
Fiduciary investment income	12	15	23	28
Operating income from continuing operations before income taxes	434	268	830	541
Interest income	4	4	10	5
Interest expense	(63)	(33)	(126)	(67)
Other income (expense)	(23)	5	(6)	12
Income from continuing operations before income taxes	352	244	708	491
Risk Solutions
Segment Reporting Information
Revenue	1,728	1,587	3,377	3,174
Commissions, fees and other revenues	1,716	1,573	3,354	3,147
Fiduciary investment income	12	14	23	27
Operating income from continuing operations before income taxes	355	305	661	562
Retail brokerage
Segment Reporting Information
Commissions, fees and other revenues	1,349	1,214	2,600	2,400
Reinsurance brokerage
Segment Reporting Information
Commissions, fees and other revenues	367	359	754	747
HR Solutions
Segment Reporting Information
Revenue	1,090	317	2,207	639
Commissions, fees and other revenues	1,090	316	2,207	638
Fiduciary investment income		1		1
Operating income from continuing operations before income taxes	115	45	238	94
Consulting services
Segment Reporting Information
Commissions, fees and other revenues	546	265	1,115	540
Outsourcing
Segment Reporting Information
Commissions, fees and other revenues	550	51	1,106	98
Intrasegment
Segment Reporting Information
Commissions, fees and other revenues	(6)		(14)
Intersegment elimination
Segment Reporting Information
Revenue	(7)	(6)	(14)	(11)
Commissions, fees and other revenues	(7)	(6)	(14)	(11)
Unallocated Expense
Segment Reporting Information
Operating income from continuing operations before income taxes	$ (36)	$ (82)	$ (69)	$ (115)

The sum of Commissions, Fees and Other Income. Commissions and fees reflects the total commissions and fees earned from providing services such as (1) brokerage and (2) other such services and activities performed for others. Other income reflects the sum of all other revenue and income recognized by the entity in the period not otherwise specified in the income statement.

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Income earned on the interest-bearing assets used as temporary investments of premiums collected from insured's but not yet remitted to insurance companies.

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- Definition

Sum of operating profit and nonoperating income (expense) before income taxes, but includes income (loss) from equity method investments.

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- Definition

The number of reportable segments of the entity.

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- Definition

The cost of borrowed funds accounted for as interest that was charged against earnings during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 34
-Paragraph 21

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher OTS
-Name Federal Regulation (FR)
-Number Title 12
-Chapter V
-Section 563c.102
-Paragraph 9
-Subsection II

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 04
-Paragraph 9
-Article 9

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- Definition

Income derived from investments in debt securities and on cash and cash equivalents the earnings of which reflect the time value of money or transactions in which the payments are for the use or forbearance of money.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 14

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 7
-Article 5

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The net result for the period of deducting operating expenses from operating revenues.

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The net amount of other nonoperating income and expense, which does not qualify for separate disclosure on the income statement under materiality guidelines.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 9
-Article 5

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Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 1
-Article 5

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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.

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