Segment Information (Details) - Information About Reportable Segments	3 Months Ended
Mar. 31, 2017 USD ($) segment	Mar. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| segment	3
Revenues	$ 625,169,000	$ 557,266,000
Operating income	53,514,000	64,429,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	0	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	625,169,000	557,266,000
Gross contribution	388,617,000	350,547,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses	(335,103,000)	(286,118,000)
Research \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	504,652,000	440,271,000
Gross contribution	346,708,000	308,186,000
Consulting \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	85,248,000	84,940,000
Gross contribution	28,342,000	29,378,000
Events \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	35,269,000	32,055,000
Gross contribution	$ 13,567,000	$ 12,983,000

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.

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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.

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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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