Segment Information (Details) - Information About Reportable Segments (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 segment	Jun. 30, 2013
Segment Reporting Information [Line Items]
Number of reportable segments			3
Revenues	$ 519,820	$ 446,047	$ 966,522	$ 852,801
Gross contribution	318,730	269,710	598,133	514,569
Corporate and other expenses	(236,969)	(195,723)	(457,202)	(386,577)
Operating income	81,761	73,987	140,931	127,992
Research
Segment Reporting Information [Line Items]
Revenues	358,495	311,233	706,609	621,564
Gross contribution	248,263	213,411	494,365	428,625
Consulting
Segment Reporting Information [Line Items]
Revenues	93,488	85,928	177,759	158,561
Gross contribution	36,235	33,185	66,573	55,722
Events
Segment Reporting Information [Line Items]
Revenues	67,837	48,886	82,154	72,676
Gross contribution	$ 34,232	$ 23,114	$ 37,195	$ 30,222

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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