Segment Information (Details) - Information About Reportable Segments $ in Thousands	3 Months Ended
Mar. 31, 2016 USD ($) segment	Mar. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| segment	3
Revenues	$ 557,266	$ 471,186
Operating income	64,429	48,682
Reportable segments
Segment Reporting Information [Line Items]
Revenues	557,266	471,186
Gross contribution	350,547	295,834
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses	(286,118)	(247,152)
Research \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	440,271	381,090
Gross contribution	308,186	267,065
Consulting \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	84,940	76,792
Gross contribution	29,378	26,327
Events \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	32,055	13,304
Gross contribution	$ 12,983	$ 2,442

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=63488584&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.

+ References

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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

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

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Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

+ 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=63488584&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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