Segment Information (Details) - Information About Reportable Segments (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 segment	Sep. 30, 2012
Segment Reporting Information [Line Items]
Number of reportable segments			3
Revenues	$ 410,705	$ 374,406	$ 1,263,506	$ 1,141,059
Gross contribution	248,538	222,431	763,108	684,300
Corporate and other expenses	(189,795)	(172,663)	(576,373)	(518,254)
Operating income	58,743	49,768	186,735	166,046
Research
Segment Reporting Information [Line Items]
Revenues	316,518	284,048	938,082	836,970
Gross contribution	220,329	193,540	648,954	571,547
Consulting
Segment Reporting Information [Line Items]
Revenues	70,149	71,731	228,710	222,970
Gross contribution	21,114	24,380	76,836	79,886
Events
Segment Reporting Information [Line Items]
Revenues	24,038	18,627	96,714	81,119
Gross contribution	$ 7,095	$ 4,511	$ 37,318	$ 32,867

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.

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- Definition

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 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=6880815&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

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