Operating Segments - Schedule of Information about Reportable Segments, Disaggregation of Revenue by Product (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 113,541	$ 128,183	$ 219,886	$ 228,832
Segment expenses	(39,175)	(51,376)	(77,337)	(91,721)
Selling, marketing, administrative and other expenses	(57,810)	(64,599)	(117,685)	(126,610)
Amortization of intangible assets	(4,713)	(5,099)	(9,425)	(11,309)
Acquisition and integration costs	(612)	(2,487)	(3,487)	(5,454)
Interest expense, other income (expense) and gains (losses) on investments	844	(2,179)	(371)	(4,837)
Income (loss) before income taxes	12,075	2,443	11,581	(11,099)
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	86,972	91,303	173,794	174,299
Segment expenses	(25,870)	(31,209)	(53,334)	(61,148)
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	21,537	18,993	40,970	36,392
Segment expenses	(10,099)	(9,474)	(20,120)	(18,908)
Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	5,032	17,887	5,122	18,141
Segment expenses	(3,206)	(10,693)	(3,883)	(11,665)
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	55,358	56,305	108,964	106,085
Research [Member] \| Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	55,358	56,305	108,964	106,085
Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	13,078	14,067	26,967	27,638
Connect [Member] \| Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	13,078	14,067	26,967	27,638
Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	5,185	5,907	10,486	11,165
Analytics [Member] \| Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	5,185	5,907	10,486	11,165
Total Research Revenues [Member]
Segment Reporting Information [Line Items]
Total revenues	73,621	76,279	146,417	144,888
Total Research Revenues [Member] \| Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	73,621	76,279	146,417	144,888
Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	34,888	34,017	68,347	65,803
Consulting [Member] \| Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	13,351	15,024	27,377	29,411
Consulting [Member] \| Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	21,537	18,993	40,970	36,392
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	5,032	17,887	5,122	18,141
Events [Member] \| Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 5,032	$ 17,887	$ 5,122	$ 18,141

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The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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The aggregate cost of goods produced and sold and services rendered during the reporting period.

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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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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The aggregate total costs related to selling a firm's product and services, as well as all other general and administrative expenses. Direct selling expenses (for example, credit, warranty, and advertising) are expenses that can be directly linked to the sale of specific products. Indirect selling expenses are expenses that cannot be directly linked to the sale of specific products, for example telephone expenses, Internet, and postal charges. General and administrative expenses include salaries of non-sales personnel, rent, utilities, communication, etc.

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