Segment Information (Details) - Information About Reportable Segments (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2014 segment	Mar. 31, 2013
Segment Reporting Information [Line Items]
Number of reportable segments	3
Revenues	$ 446,702	$ 406,754
Gross contribution	279,402	244,860
Corporate and other expenses	(220,232)	(190,855)
Operating income	59,170	54,005
Research
Segment Reporting Information [Line Items]
Revenues	348,114	310,331
Gross contribution	246,101	215,214
Consulting
Segment Reporting Information [Line Items]
Revenues	84,271	72,633
Gross contribution	30,338	22,538
Events
Segment Reporting Information [Line Items]
Revenues	14,317	23,790
Gross contribution	$ 2,963	$ 7,108

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

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- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

No definition available.

+ Details

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- Definition

Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

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)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

+ Details

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- Details

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