Segment Information - Information About Reportable Segments (Details)	3 Months Ended
Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments \| segment	3
Number of reportable segments \| segment	3
Operating income (loss)	$ 48,799,000	$ (8,711,000)
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	970,444,000	963,565,000
Gross contribution	622,762,000	614,814,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses	(573,963,000)	(623,525,000)
Research \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	825,374,000	763,924,000
Gross contribution	575,168,000	531,456,000
Conferences \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	51,932,000	46,087,000
Gross contribution	18,876,000	16,190,000
Consulting \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	93,138,000	82,896,000
Gross contribution	$ 28,718,000	24,124,000
Other \| Reportable segments
Segment Reporting Information [Line Items]
Revenues		70,658,000
Gross contribution		$ 43,044,000

Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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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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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).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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