Segment Information (Details) - Information About Reportable Segments (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 segment	Sep. 30, 2013
Segment Reporting Information [Line Items]
Number of reportable segments			3
Revenues	$ 470,940	$ 410,705	$ 1,437,462	$ 1,263,506
Gross contribution	282,833	248,538	880,965	763,108
Corporate and other expenses	(233,442)	(189,795)	(690,643)	(576,373)
Operating income	49,391	58,743	190,322	186,735
Research
Segment Reporting Information [Line Items]
Revenues	365,334	316,518	1,071,943	938,082
Gross contribution	251,113	220,329	745,477	648,954
Consulting
Segment Reporting Information [Line Items]
Revenues	82,300	70,149	260,059	228,710
Gross contribution	24,774	21,114	91,347	76,836
Events
Segment Reporting Information [Line Items]
Revenues	23,306	24,038	105,460	96,714
Gross contribution	$ 6,946	$ 7,095	$ 44,141	$ 37,318

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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