Segment Information - Information About Reportable Segments (Details)	3 Months Ended
Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments \| segment	4
Number of reportable segments \| segment	4
Revenues	$ 963,565,000	$ 625,169,000
Operating (loss) income	(8,711,000)	53,514,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	963,565,000	625,169,000
Gross contribution	614,814,000	388,617,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses	(623,525,000)	(335,103,000)
Research
Segment Reporting Information [Line Items]
Revenues		6,700,000
Gross contribution		4,400,000
Research \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	763,924,000	511,306,000
Gross contribution	531,456,000	351,113,000
Events \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	46,087,000	35,269,000
Gross contribution	16,190,000	13,567,000
Consulting
Segment Reporting Information [Line Items]
Revenues		(6,700,000)
Gross contribution		(4,400,000)
Consulting \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	82,896,000	78,594,000
Gross contribution	24,124,000	23,937,000
Talent Assessment & Other \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	70,658,000	0
Gross contribution	$ 43,044,000	$ 0

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 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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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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