Information about the Reportable Segments (Detail) (USD $) In Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Revenues	$ 345,784	$ 296,122	$ 1,040,894	$ 906,150
Gross contribution	203,626	170,560	613,773	522,221
Corporate and other expenses	(156,376)	(137,797)	(469,174)	(426,030)
Operating income	47,250	32,763	144,599	96,191
Interest expense, net	(2,282)	(3,005)	(7,863)	(9,569)
Other expense, net	(541)	(373)	(1,494)	736
Income before income taxes	44,427	29,385	135,242	87,358
Research
Segment Reporting Information [Line Items]
Revenues	255,979	214,680	749,429	634,448
Gross contribution	173,615	140,605	506,420	415,311
Consulting
Segment Reporting Information [Line Items]
Revenues	70,815	65,397	219,407	212,796
Gross contribution	24,458	23,981	78,820	84,222
Events
Segment Reporting Information [Line Items]
Revenues	18,990	16,045	72,058	58,906
Gross contribution	$ 5,553	$ 5,974	$ 28,533	$ 22,688

Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1,2)

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Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 4

-Section 08

-Paragraph h

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

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

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

-Publisher FASB

-Name Accounting Standards Codification

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Name Federal Regulation (FR)

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-Chapter V

-Section 563c.102

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-LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy. It will be removed from future versions of this taxonomy.

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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.

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