Segment Information (Details) - Information About Reportable Segments $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) segment	Jun. 30, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| segment			3
Operating income	$ 85,220	$ 81,761	$ 133,902	$ 140,931
Reportable segments
Segment Reporting Information [Line Items]
Revenues	547,936	519,820	1,019,122	966,522
Gross contribution	342,543	318,730	638,377	598,133
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses	(257,323)	(236,969)	(504,475)	(457,202)
Research \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	385,718	358,495	766,808	706,609
Gross contribution	269,846	248,263	536,911	494,365
Consulting \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	88,336	93,488	165,128	177,759
Gross contribution	33,255	36,235	59,582	66,573
Events \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	73,882	67,837	87,186	82,154
Gross contribution	$ 39,442	$ 34,232	$ 41,884	$ 37,195

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)

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

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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.

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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

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

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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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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